Fix loading custom template translations for en

The English language was hardcoded to load empty translations for
templates as part of sphinx-doc#10067,
but the rest of the code is already capable of handling missing
translations just fine, so we can simply drop the first if block.

This allows to use translations for English in custom templates, as for
all the other languages.

Fixes sphinx-doc#12214

sphinx/application.py

@@ -271,29 +271,26 @@ def _init_i18n(self) -> None:
         """Load translated strings from the configured localedirs if enabled in
         the configuration.
         """
-        if self.config.language == 'en':
-            self.translator, _ = locale.init([], None)
+        logger.info(bold(__('loading translations [%s]... ') % self.config.language),
+                    nonl=True)
+
+        # compile mo files if sphinx.po file in user locale directories are updated
+        repo = CatalogRepository(self.srcdir, self.config.locale_dirs,
+                                 self.config.language, self.config.source_encoding)
+        for catalog in repo.catalogs:
+            if catalog.domain == 'sphinx' and catalog.is_outdated():
+                catalog.write_mo(self.config.language,
+                                 self.config.gettext_allow_fuzzy_translations)
+
+        locale_dirs: list[str | None] = list(repo.locale_dirs)
+        locale_dirs += [None]
+        locale_dirs += [path.join(package_dir, 'locale')]
+
+        self.translator, has_translation = locale.init(locale_dirs, self.config.language)
+        if has_translation:
+            logger.info(__('done'))
         else:
-            logger.info(bold(__('loading translations [%s]... ') % self.config.language),
-                        nonl=True)
-
-            # compile mo files if sphinx.po file in user locale directories are updated
-            repo = CatalogRepository(self.srcdir, self.config.locale_dirs,
-                                     self.config.language, self.config.source_encoding)
-            for catalog in repo.catalogs:
-                if catalog.domain == 'sphinx' and catalog.is_outdated():
-                    catalog.write_mo(self.config.language,
-                                     self.config.gettext_allow_fuzzy_translations)
-
-            locale_dirs: list[str | None] = list(repo.locale_dirs)
-            locale_dirs += [None]
-            locale_dirs += [path.join(package_dir, 'locale')]
-
-            self.translator, has_translation = locale.init(locale_dirs, self.config.language)
-            if has_translation:
-                logger.info(__('done'))
-            else:
-                logger.info(__('not available for built-in messages'))
+            logger.info(__('not available for built-in messages'))
 
     def _init_env(self, freshenv: bool) -> BuildEnvironment:
         filename = path.join(self.doctreedir, ENV_PICKLE_FILENAME)