fix(i18n): return the correct locale from filenames (#5922)

packages/netlify-cms-core/src/lib/__tests__/i18n.spec.js

@@ -207,6 +207,36 @@ describe('i18n', () => {
     });
   });
 
+  describe('getLocaleFromPath', () => {
+    it('should return the locale from folder name in the path when structure is I18N_STRUCTURE.MULTIPLE_FOLDERS', () => {
+      expect(
+        i18n.getLocaleFromPath(
+          i18n.I18N_STRUCTURE.MULTIPLE_FOLDERS,
+          'md',
+          'src/content/en/index.md',
+        ),
+      ).toEqual('en');
+    });
+
+    it('should return the locale extension from the file name when structure is I18N_STRUCTURE.MULTIPLE_FILES', () => {
+      expect(
+        i18n.getLocaleFromPath(i18n.I18N_STRUCTURE.MULTIPLE_FILES, 'md', 'src/content/index.en.md'),
+      ).toEqual('en');
+    });
+
+    it('issue #5909: return the correct locale extension for language gd', () => {
+      expect(
+        i18n.getLocaleFromPath(i18n.I18N_STRUCTURE.MULTIPLE_FILES, 'md', 'src/content/index.gd.md'),
+      ).toEqual('gd');
+    });
+
+    it('should return an empty string when structure is I18N_STRUCTURE.SINGLE_FILE', () => {
+      expect(
+        i18n.getLocaleFromPath(i18n.I18N_STRUCTURE.SINGLE_FILE, 'md', 'src/content/index.md'),
+      ).toEqual('');
+    });
+  });
+
   describe('getI18nFiles', () => {
     const locales = ['en', 'de', 'fr'];
     const default_locale = 'en';

packages/netlify-cms-core/src/lib/i18n.ts

@@ -1,5 +1,5 @@
 import { Map, List } from 'immutable';
-import { set, trimEnd, groupBy, escapeRegExp } from 'lodash';
+import { set, groupBy, escapeRegExp } from 'lodash';
 
 import { selectEntrySlug } from '../reducers/collections';
 
@@ -98,7 +98,7 @@ export function getLocaleFromPath(structure: I18N_STRUCTURE, extension: string,
       return parts.pop();
     }
     case I18N_STRUCTURE.MULTIPLE_FILES: {
-      const parts = trimEnd(path, `.${extension}`);
+      const parts = path.slice(0, -`.${extension}`.length);
       return parts.split('.').pop();
     }
     case I18N_STRUCTURE.SINGLE_FILE: