Default to english when no lang is slected (#1396)

Fixes https://github.com/calibrain/shelfmark/issues/1384

src/frontend/src/tests/languageFilters.test.ts

@@ -5,6 +5,7 @@ import {
   LANGUAGE_OPTION_ALL,
   LANGUAGE_OPTION_DEFAULT,
   buildLanguageNormalizer,
+  getLanguageFilterValues,
   getReleaseSearchLanguageParams,
   releaseLanguageMatchesFilter,
   resolveDefaultLanguageCodes,
@@ -67,6 +68,25 @@ describe('releaseLanguageMatchesFilter', () => {
 
     expect(visibleLanguages).toHaveLength(48);
   });
+
+  it('shows every release when the default language list is explicitly empty', () => {
+    // Issue 1384: an empty BOOK_LANGUAGE with the "Default" filter hid all releases.
+    const normalizer = buildLanguageNormalizer(supportedLanguages);
+    const defaults = resolveDefaultLanguageCodes([], supportedLanguages);
+    const selectedCodes =
+      getLanguageFilterValues([LANGUAGE_OPTION_DEFAULT], supportedLanguages, defaults) ?? defaults;
+
+    expect(releaseLanguageMatchesFilter('en', selectedCodes, normalizer)).toBe(true);
+    expect(releaseLanguageMatchesFilter('German', selectedCodes, normalizer)).toBe(true);
+    expect(releaseLanguageMatchesFilter('de, en', selectedCodes, normalizer)).toBe(true);
+  });
+
+  it('still filters by a configured default language', () => {
+    const normalizer = buildLanguageNormalizer(supportedLanguages);
+
+    expect(releaseLanguageMatchesFilter('en', ['en'], normalizer)).toBe(true);
+    expect(releaseLanguageMatchesFilter('de', ['en'], normalizer)).toBe(false);
+  });
 });
 
 describe('resolveDefaultLanguageCodes', () => {

src/frontend/src/utils/languageFilters.ts

@@ -123,7 +123,7 @@ export const buildLanguageNormalizer = (languages: Language[]): Map<string, stri
  * (e.g., "English, Spanish", "English/Spanish", "English + Spanish", "English & Spanish").
  *
  * @param releaseLang - Language string from the release (can be code or full name)
- * @param selectedCodes - Array of selected ISO language codes
+ * @param selectedCodes - Array of selected ISO language codes; null or empty means no filter
  * @param languageNormalizer - Optional map to normalize language names to codes
  */
 export const releaseLanguageMatchesFilter = (
@@ -131,7 +131,9 @@ export const releaseLanguageMatchesFilter = (
   selectedCodes: string[] | null,
   languageNormalizer?: Map<string, string>,
 ): boolean => {
-  if (!releaseLang || !selectedCodes) {
+  // An empty list is an explicitly empty BOOK_LANGUAGE ("no default filter"), which the
+  // backend does not filter on either; treating it as "match nothing" hid every release.
+  if (!releaseLang || !selectedCodes || selectedCodes.length === 0) {
     return true;
   }
   if (selectedCodes.includes(LANGUAGE_OPTION_ALL)) {