I'm currently developing an Android app with Appcelerator which is only in Spanish.
My problem is that if the user doesn't have its smartphone in Spanish, strings from strings.xml won't show:
How do I set Spanish as the default language of my app? I found a solution on the Appcelerator wiki, but it's just for iOS https://wiki.appcelerator.org/display/guides2/Internationalization#Internationalization-Setdefaultlanguage
English is the default translation for all Titanium apps, therefore the English files are mandatory. If you just add the English folder to your i18n directory (inside the app directory) you should be good.
If your app is only going to be in Spanish, just add Spanish to that directory and everything should work everywhere.
There is a ticket in the Appcelerator JIRA for adding support, there is also a way to override in the comments: https://jira.appcelerator.org/browse/TIMOB-20527