Restore fallback for non-regional translation

Also adds an explicit check for en given this the base translation for the application and will never have a .mo/.gmo file.

Source/options.cpp

@@ -997,6 +997,25 @@ void OptionEntryLanguageCode::LoadFromIni(string_view category)
 		}
 	}
 
+	std::vector<std::string> fallbackLocales;
+	fallbackLocales.reserve(locales.size());
+
+	for (const auto &locale : locales) {
+		std::string neutralLocale = locale.substr(0, locale.find('_'));
+		if (std::find(fallbackLocales.cbegin(), fallbackLocales.cend(), neutralLocale) == fallbackLocales.cend()) {
+			fallbackLocales.push_back(neutralLocale);
+		}
+	}
+
+	for (const auto &fallbackLocale : fallbackLocales) {
+		LogVerbose("Trying to load fallback translation: {}", fallbackLocale);
+		if (HasTranslation(fallbackLocale)) {
+			LogVerbose("Found matching fallback locale: {}", fallbackLocale);
+			CopyUtf8(szCode, fallbackLocale, sizeof(szCode));
+			return;
+		}
+	}
+
 	LogVerbose("No suitable translation found");
 	strcpy(szCode, "en");
 }

Source/utils/language.cpp

@@ -270,15 +270,19 @@ const std::string &LanguageTranslate(const char *key)
 
 bool HasTranslation(const std::string &locale)
 {
-	for (const char *ext : { ".mo", ".gmo" }) {
-		SDL_RWops *rw = OpenAsset((locale + ext).c_str());
+	if (locale == "en") {
+		return true;
+	}
+
+	constexpr std::array<char *, 2> Extensions { ".mo", ".gmo" };
+	return std::any_of(Extensions.cbegin(), Extensions.cend(), [locale](const std::string &extension) {
+		SDL_RWops *rw = OpenAsset((locale + extension).c_str());
 		if (rw != nullptr) {
 			SDL_RWclose(rw);
 			return true;
 		}
-	}
-
-	return false;
+		return false;
+	});
 }
 
 bool IsSmallFontTall()