While your app can do well by shipping an English-only, localization can improve your reach when you ship your app worldwide.
After English-only apps, the most common package of languages apps localize for are: EFIGS.
Out of these 5 languages, the one design should be the most attentive to is German, as word lengths in German can commonly be double the length of the same word in English. This can create space issues in Titlebars and abbreviations.
The selected languages will be added to supported language codes on brainCloud server, then you can retrieve the language code base on the different regions and apply the according current language in your app.
Note, user languageCode can be retrieved via PlayerState service ReadUserState API call.