I know that it is possible to have different languages in the res/values-ISOCode direcotry. Is this possible for xml files in the assets directory as well? How would the folder structure have to look?
Is this possible for xml files in the assets directory as well?
Yes and no.
Yes, you can put whatever files you want into assets/, in whatever languages you want.
No, insofar as Android does not automatically load a specific asset based on language. You would have to do that yourself (e.g., look up the language via Locale, then generate the right directory name based on that language). Android only loads things from different directories based on configuration for resources, not assets.
How would the folder structure have to look?
Whatever your generate-a-directory-name-based-on-language algorithm would require.