App localization is important, but tedious to test. One quick way is test localizations in the simulator, using launch arguments. In Xcode, click on Edit Scheme > Run > Arguments and add the following to Arguments Passed On Launch:
en with the localization (
es, etc) you want to test e.g.
Simplified Chinese: -AppleLanguages (zh) German: -AppleLanguages (de) French: -AppleLanguages (fr)