diff --git a/src/ngx-translate-loaders/theme-i18n.util.spec.ts b/src/ngx-translate-loaders/theme-i18n.util.spec.ts
new file mode 100644
index 00000000000..5d17d0c9adc
--- /dev/null
+++ b/src/ngx-translate-loaders/theme-i18n.util.spec.ts
@@ -0,0 +1,68 @@
+import { NamedThemeConfig } from '@dspace/config/theme.config';
+
+import { resolveActiveThemeChain } from './theme-i18n.util';
+
+describe('resolveActiveThemeChain', () => {
+  const theme = (name: string, ext?: string): NamedThemeConfig =>
+    ext ? { name, extends: ext } : { name };
+
+  it('returns an empty array when activeName is empty', () => {
+    expect(resolveActiveThemeChain([theme('custom')], '')).toEqual([]);
+  });
+
+  it('returns [active] when the active theme has no extends', () => {
+    expect(resolveActiveThemeChain([theme('custom')], 'custom')).toEqual(['custom']);
+  });
+
+  it('returns [parent, active] when active extends a configured parent', () => {
+    expect(resolveActiveThemeChain(
+      [theme('custom'), theme('child-theme', 'custom')],
+      'child-theme',
+    )).toEqual(['custom', 'child-theme']);
+  });
+
+  it('resolves the chain even when the active theme is listed before its parent', () => {
+    expect(resolveActiveThemeChain(
+      [theme('child-theme', 'parent-theme'), theme('parent-theme')],
+      'child-theme',
+    )).toEqual(['parent-theme', 'child-theme']);
+  });
+
+  it('expands a multi-level chain (root → mid → active)', () => {
+    expect(resolveActiveThemeChain(
+      [theme('child-theme', 'parent-theme'), theme('parent-theme', 'base-theme'), theme('base-theme')],
+      'child-theme',
+    )).toEqual(['base-theme', 'parent-theme', 'child-theme']);
+  });
+
+  it('includes an ancestor even when it is not separately listed in configured themes', () => {
+    expect(resolveActiveThemeChain(
+      [theme('child-theme', 'parent-theme')],
+      'child-theme',
+    )).toEqual(['parent-theme', 'child-theme']);
+  });
+
+  it('does NOT include sibling themes — only the active chain', () => {
+    // sibling-theme and child-theme both extend parent-theme; when child-theme is active,
+    // sibling-theme must not appear in the chain.
+    const chain = resolveActiveThemeChain(
+      [theme('parent-theme'), theme('child-theme', 'parent-theme'), theme('sibling-theme', 'parent-theme')],
+      'child-theme',
+    );
+    expect(chain).toEqual(['parent-theme', 'child-theme']);
+    expect(chain).not.toContain('sibling-theme');
+  });
+
+  it('terminates and includes each theme once when extends forms a cycle', () => {
+    const order = resolveActiveThemeChain(
+      [theme('theme-a', 'theme-b'), theme('theme-b', 'theme-a')],
+      'theme-a',
+    );
+    expect(order.length).toBe(2);
+    expect([...order].sort()).toEqual(['theme-a', 'theme-b']);
+  });
+
+  it('returns [active] when activeName does not match any configured theme', () => {
+    expect(resolveActiveThemeChain([theme('custom')], 'unknown-theme')).toEqual(['unknown-theme']);
+  });
+});
diff --git a/src/ngx-translate-loaders/theme-i18n.util.ts b/src/ngx-translate-loaders/theme-i18n.util.ts
new file mode 100644
index 00000000000..6f860025660
--- /dev/null
+++ b/src/ngx-translate-loaders/theme-i18n.util.ts
@@ -0,0 +1,44 @@
+import { ThemeConfig } from '@dspace/config/theme.config';
+
+/**
+ * Resolve the i18n load order for a single **active** theme, walking up its
+ * `extends` chain so ancestors are loaded before descendants (ancestor keys
+ * are overridden by descendant keys).
+ *
+ * Only the active theme's inheritance chain is returned.  Sibling themes that
+ * are configured but not active are intentionally excluded — loading all
+ * configured themes caused cross-theme key pollution (a sibling theme's
+ * translations silently overriding the active theme's keys).
+ *
+ * This mirrors the official `merge-i18n` CLI approach, which merges exactly
+ * one theme at a time: `npm run merge-i18n -- -s src/themes/<active>/assets/i18n`.
+ *
+ * `extends` cycles are handled gracefully (each theme name appears at most once).
+ *
+ * Examples (config `[base-theme, child-theme (extends base-theme), sibling-theme (extends base-theme)]`):
+ *   active = 'base-theme'    (no extends)                  → ['base-theme']
+ *   active = 'child-theme'   (extends: 'base-theme')       → ['base-theme', 'child-theme']
+ *   active = 'sibling-theme' (extends: 'base-theme')       → ['base-theme', 'sibling-theme']
+ *
+ * @param configured the configured themes (typically `environment.themes`)
+ * @param activeName the name of the active/default theme
+ * @returns ordered list of theme names to load (root ancestor → active theme)
+ */
+export function resolveActiveThemeChain(configured: ThemeConfig[], activeName: string): string[] {
+  const byName = new Map<string, ThemeConfig>();
+  configured.forEach((t) => {
+    if (t?.name) {
+      byName.set(t.name, t);
+    }
+  });
+
+  const chain: string[] = [];
+  const visited = new Set<string>();
+  let cursor: string | undefined = activeName;
+  while (cursor && !visited.has(cursor)) {
+    visited.add(cursor);
+    chain.push(cursor);
+    cursor = byName.get(cursor)?.extends;
+  }
+  return chain.reverse(); // ancestor → descendant
+}
diff --git a/src/ngx-translate-loaders/translate-browser.loader.spec.ts b/src/ngx-translate-loaders/translate-browser.loader.spec.ts
new file mode 100644
index 00000000000..02e5e42d625
--- /dev/null
+++ b/src/ngx-translate-loaders/translate-browser.loader.spec.ts
@@ -0,0 +1,174 @@
+import {
+  of,
+  throwError,
+} from 'rxjs';
+
+import { environment } from '../environments/environment';
+import { TranslateBrowserLoader } from './translate-browser.loader';
+
+describe('TranslateBrowserLoader', () => {
+  const PREFIX = 'assets/i18n/';
+  const SUFFIX = '.json';
+
+  let transferState: { get: jasmine.Spy; set: jasmine.Spy };
+  let http: { get: jasmine.Spy };
+  let loader: TranslateBrowserLoader;
+  let originalThemes: typeof environment.themes;
+
+  /** Match the base i18n request regardless of the production content hash. */
+  const isBaseRequest = (url: string): boolean => url.startsWith(`${PREFIX}en`) && url.endsWith(SUFFIX);
+
+  beforeEach(() => {
+    transferState = {
+      get: jasmine.createSpy('get'),
+      set: jasmine.createSpy('set'),
+    };
+    http = { get: jasmine.createSpy('get') };
+    loader = new TranslateBrowserLoader(transferState as any, http as any, PREFIX, SUFFIX);
+    originalThemes = environment.themes;
+  });
+
+  afterEach(() => {
+    environment.themes = originalThemes;
+  });
+
+  it('returns the cached translations from the TransferState without any HTTP call', (done) => {
+    const cached = { 'home.title': 'Cached' };
+    transferState.get.and.returnValue({ en: cached });
+
+    loader.getTranslation('en').subscribe((messages) => {
+      expect(messages).toEqual(cached);
+      expect(http.get).not.toHaveBeenCalled();
+      done();
+    });
+  });
+
+  it('fetches the base file and merges theme overrides on top (theme keys win)', (done) => {
+    environment.themes = [{ name: 'custom' }];
+    transferState.get.and.returnValue({});
+    http.get.and.callFake((url: string) => {
+      if (isBaseRequest(url)) {
+        return of(JSON.stringify({ 'home.title': 'Base', 'home.subtitle': 'Base sub' }));
+      }
+      if (url === 'assets/custom/i18n/en.json5') {
+        return of('{ "home.title": "Custom" }');
+      }
+      return throwError(() => new Error('404'));
+    });
+
+    loader.getTranslation('en').subscribe((messages) => {
+      expect(messages).toEqual({ 'home.title': 'Custom', 'home.subtitle': 'Base sub' });
+      done();
+    });
+  });
+
+  it('falls back to the base translations when a theme override file is missing', (done) => {
+    environment.themes = [{ name: 'custom' }];
+    transferState.get.and.returnValue({});
+    http.get.and.callFake((url: string) => {
+      if (isBaseRequest(url)) {
+        return of(JSON.stringify({ 'home.title': 'Base' }));
+      }
+      return throwError(() => new Error('404'));
+    });
+
+    loader.getTranslation('en').subscribe((messages) => {
+      expect(messages).toEqual({ 'home.title': 'Base' });
+      done();
+    });
+  });
+
+  it('applies overrides in inheritance order so descendant themes win over ancestors', (done) => {
+    environment.themes = [{ name: 'child-theme', extends: 'parent-theme' }, { name: 'parent-theme' }];
+    transferState.get.and.returnValue({});
+    http.get.and.callFake((url: string) => {
+      if (isBaseRequest(url)) {
+        return of(JSON.stringify({ key: 'base' }));
+      }
+      if (url === 'assets/parent-theme/i18n/en.json5') {
+        return of('{ key: "parent" }');
+      }
+      if (url === 'assets/child-theme/i18n/en.json5') {
+        return of('{ key: "child" }');
+      }
+      return throwError(() => new Error('404'));
+    });
+
+    loader.getTranslation('en').subscribe((messages) => {
+      expect(messages.key).toBe('child');
+      done();
+    });
+  });
+
+  it('returns the base translations unchanged when no themes are configured', (done) => {
+    environment.themes = [];
+    transferState.get.and.returnValue({});
+    http.get.and.callFake((url: string) => {
+      if (isBaseRequest(url)) {
+        return of(JSON.stringify({ 'home.title': 'Base' }));
+      }
+      return throwError(() => new Error('404'));
+    });
+
+    loader.getTranslation('en').subscribe((messages) => {
+      expect(messages).toEqual({ 'home.title': 'Base' });
+      done();
+    });
+  });
+
+  it('does NOT apply sibling theme overrides when the active theme is first in the list', (done) => {
+    // Regression: when parent-theme is active, a sibling theme that also extends parent-theme
+    // must NOT override parent-theme's translations.
+    environment.themes = [
+      { name: 'parent-theme' },
+      { name: 'sibling-theme', extends: 'parent-theme' },
+    ];
+    transferState.get.and.returnValue({});
+    http.get.and.callFake((url: string) => {
+      if (isBaseRequest(url)) {
+        return of(JSON.stringify({ key: 'base' }));
+      }
+      if (url === 'assets/parent-theme/i18n/en.json5') {
+        return of('{ key: "parent" }');
+      }
+      if (url === 'assets/sibling-theme/i18n/en.json5') {
+        // sibling has its own value for the same key — must NOT win when parent-theme is active
+        return of('{ key: "sibling" }');
+      }
+      return throwError(() => new Error('404'));
+    });
+
+    loader.getTranslation('en').subscribe((messages) => {
+      expect(messages.key).toBe('parent');
+      done();
+    });
+  });
+
+  it('applies the full ancestor chain when a child theme is active', (done) => {
+    // When child-theme (extends parent-theme) is active, parent-theme keys provide the base
+    // and child-theme keys override only what they define.
+    environment.themes = [
+      { name: 'child-theme', extends: 'parent-theme' },
+      { name: 'parent-theme' },
+    ];
+    transferState.get.and.returnValue({});
+    http.get.and.callFake((url: string) => {
+      if (isBaseRequest(url)) {
+        return of(JSON.stringify({ 'key.a': 'base-a', 'key.b': 'base-b' }));
+      }
+      if (url === 'assets/parent-theme/i18n/en.json5') {
+        return of('{ "key.a": "parent-a" }');
+      }
+      if (url === 'assets/child-theme/i18n/en.json5') {
+        return of('{ "key.b": "child-b" }');
+      }
+      return throwError(() => new Error('404'));
+    });
+
+    loader.getTranslation('en').subscribe((messages) => {
+      expect(messages['key.a']).toBe('parent-a');  // inherited from parent-theme
+      expect(messages['key.b']).toBe('child-b');   // overridden by child-theme
+      done();
+    });
+  });
+});
diff --git a/src/ngx-translate-loaders/translate-browser.loader.ts b/src/ngx-translate-loaders/translate-browser.loader.ts
index 44e1710b148..efbd109c895 100644
--- a/src/ngx-translate-loaders/translate-browser.loader.ts
+++ b/src/ngx-translate-loaders/translate-browser.loader.ts
@@ -1,22 +1,37 @@
 import { HttpClient } from '@angular/common/http';
 import { TransferState } from '@angular/core';
+import { AppConfig } from '@dspace/config/app-config.interface';
+import { getDefaultThemeConfig } from '@dspace/config/config.util';
 import { hasValue } from '@dspace/shared/utils/empty.util';
 import { TranslateLoader } from '@ngx-translate/core';
+import JSON5 from 'json5';
 import {
+  forkJoin,
   Observable,
   of,
 } from 'rxjs';
-import { map } from 'rxjs/operators';
+import {
+  catchError,
+  map,
+} from 'rxjs/operators';
 
 import { environment } from '../environments/environment';
 import {
   NGX_TRANSLATE_STATE,
   NgxTranslateState,
 } from './ngx-translate-state';
+import { resolveActiveThemeChain } from './theme-i18n.util';
 
 /**
  * A TranslateLoader for ngx-translate to retrieve i18n messages from the TransferState, or download
- * them if they're not available there
+ * them if they're not available there.
+ *
+ * Also fetches per-theme translation overrides from `assets/<theme>/i18n/<lang>.json5` for the
+ * active theme and its ancestors (root → active), merging them on top of the base translations
+ * so the active theme's keys win. Only the active theme's inheritance chain is loaded — sibling
+ * themes are excluded to prevent cross-theme key pollution.
+ * This removes the need for the build-time `merge-i18n` script, allowing a single Docker image
+ * to serve multiple customers with their own theme-specific translations.
  */
 export class TranslateBrowserLoader implements TranslateLoader {
   constructor(
@@ -35,18 +50,46 @@ export class TranslateBrowserLoader implements TranslateLoader {
    */
   getTranslation(lang: string): Observable<any> {
     // Get the ngx-translate messages from the transfer state, to speed up the initial page load
-    // client side
+    // client side. The server has already merged theme overrides into this state.
     const state = this.transferState.get<NgxTranslateState>(NGX_TRANSLATE_STATE, {});
     const messages = state[lang];
     if (hasValue(messages)) {
       return of(messages);
-    } else {
-      const translationHash: string = environment.production ? `.${(process.env.languageHashes as any)[lang + '.json5']}` : '';
-      // If they're not available on the transfer state (e.g. when running in dev mode), retrieve
-      // them using HttpClient
-      return this.http.get(`${this.prefix}${lang}${translationHash}${this.suffix}`, { responseType: 'text' }).pipe(
-        map((json: any) => JSON.parse(json)),
-      );
     }
+
+    // Fetch base translations + the active theme's override files (root ancestor → active theme).
+    // Only the active theme's inheritance chain is loaded; sibling themes are excluded.
+    const base$ = this.fetchBase(lang);
+    const activeName = getDefaultThemeConfig(environment as unknown as AppConfig).name;
+    const orderedThemes = resolveActiveThemeChain(environment.themes ?? [], activeName);
+    const themeStreams = orderedThemes.map((name: string) => this.fetchThemeOverride(name, lang));
+
+    return forkJoin([base$, ...themeStreams]).pipe(
+      map((parts) => parts.reduce((acc, part) => Object.assign(acc, part), {})),
+    );
+  }
+
+  /**
+   * Fetch the base i18n file (the hashed JSON produced by the build pipeline).
+   */
+  protected fetchBase(lang: string): Observable<Record<string, string>> {
+    const translationHash: string = environment.production
+      ? `.${(process.env.languageHashes as any)[lang + '.json5']}`
+      : '';
+    return this.http.get(`${this.prefix}${lang}${translationHash}${this.suffix}`, { responseType: 'text' }).pipe(
+      map((json: any) => JSON.parse(json)),
+      catchError(() => of({})),
+    );
+  }
+
+  /**
+   * Fetch the override JSON5 file for the given theme/language.
+   * Returns an empty object if the file is missing (most themes won't override every language).
+   */
+  protected fetchThemeOverride(themeName: string, lang: string): Observable<Record<string, string>> {
+    return this.http.get(`assets/${themeName}/i18n/${lang}.json5`, { responseType: 'text' }).pipe(
+      map((text: string) => JSON5.parse(text) as Record<string, string>),
+      catchError(() => of({})),
+    );
   }
 }
diff --git a/src/ngx-translate-loaders/translate-server.loader.ts b/src/ngx-translate-loaders/translate-server.loader.ts
index 06312b3a2b0..82b331bd4c0 100644
--- a/src/ngx-translate-loaders/translate-server.loader.ts
+++ b/src/ngx-translate-loaders/translate-server.loader.ts
@@ -1,20 +1,39 @@
-import { readFileSync } from 'node:fs';
+import {
+  existsSync,
+  readFileSync,
+} from 'node:fs';
+import {
+  dirname,
+  resolve,
+} from 'node:path';
 
 import { TransferState } from '@angular/core';
+import { AppConfig } from '@dspace/config/app-config.interface';
+import { getDefaultThemeConfig } from '@dspace/config/config.util';
 import { TranslateLoader } from '@ngx-translate/core';
+import JSON5 from 'json5';
 import {
   Observable,
   of,
 } from 'rxjs';
 
+import { environment } from '../environments/environment';
 import {
   NGX_TRANSLATE_STATE,
   NgxTranslateState,
 } from './ngx-translate-state';
+import { resolveActiveThemeChain } from './theme-i18n.util';
 
 /**
  * A TranslateLoader for ngx-translate to parse json5 files server-side, and store them in the
- * TransferState
+ * TransferState.
+ *
+ * Also overlays per-theme translation overrides from `<themeAssetsRoot>/<theme>/i18n/<lang>.json5`
+ * for the active theme and its ancestors (in root → active order), merging them on top of the
+ * base translations so the active theme's keys win. Only the active theme's inheritance chain is
+ * loaded — sibling themes are excluded to prevent cross-theme key pollution.
+ * The resulting merged map is stored in the TransferState so the browser does not need to
+ * re-fetch the theme overrides after the initial SSR response.
  */
 export class TranslateServerLoader implements TranslateLoader {
 
@@ -32,13 +51,56 @@ export class TranslateServerLoader implements TranslateLoader {
    */
   public getTranslation(lang: string): Observable<any> {
     const translationHash: string = (process.env.languageHashes as any)[lang + '.json5'];
-    // Retrieve the file for the given language, and parse it
-    const messages = JSON.parse(readFileSync(`${this.prefix}${lang}.${translationHash}${this.suffix}`, 'utf8'));
-    // Store the parsed messages in the transfer state so they'll be available immediately when the
-    // app loads on the client
-    this.storeInTransferState(lang, messages);
-    // Return the parsed messages to translate things server side
-    return of(messages);
+    // Retrieve the base file for the given language and parse it
+    const baseMessages = JSON.parse(readFileSync(`${this.prefix}${lang}.${translationHash}${this.suffix}`, 'utf8'));
+
+    // Overlay theme override files (if any). Theme keys override base keys.
+    const merged = Object.assign({}, baseMessages, this.readThemeOverrides(lang));
+
+    // Store the parsed messages in the transfer state so they'll be available immediately when
+    // the app loads on the client
+    this.storeInTransferState(lang, merged);
+    // Return the merged messages to translate things server side
+    return of(merged);
+  }
+
+  /**
+   * Read and merge i18n override files for the active theme's inheritance chain.
+   * Only the active theme (and its ancestors) are loaded; sibling themes are excluded.
+   * Returns an empty object if no overrides are present.
+   *
+   * @param lang the language code
+   * @protected
+   */
+  protected readThemeOverrides(lang: string): Record<string, string> {
+    const configured = environment.themes ?? [];
+    if (configured.length === 0) {
+      return {};
+    }
+
+    // Resolve only the active theme's chain (root ancestor → active theme).
+    const activeName = getDefaultThemeConfig(environment as unknown as AppConfig).name;
+    const orderedThemes = resolveActiveThemeChain(configured, activeName);
+
+    // `this.prefix` looks like `dist/server/assets/i18n/`. Theme assets live next to `i18n/`
+    // at `dist/server/assets/<theme>/i18n/<lang>.json5`.
+    const assetsRoot = dirname(this.prefix.replace(/\/$/, ''));
+
+    const overrides: Record<string, string> = {};
+    for (const theme of orderedThemes) {
+      const filePath = resolve(`${assetsRoot}/${theme}/i18n/${lang}.json5`);
+      if (existsSync(filePath)) {
+        try {
+          const themeMessages = JSON5.parse(readFileSync(filePath, 'utf8')) as Record<string, string>;
+          Object.assign(overrides, themeMessages);
+        } catch (e) {
+          // Skip malformed theme i18n files so they don't break SSR
+          // eslint-disable-next-line no-console
+          console.warn(`[TranslateServerLoader] Failed to parse theme i18n file ${filePath}:`, e);
+        }
+      }
+    }
+    return overrides;
   }
 
   /**