Translation interface for multilanguage UI

There are many users who would like to use the app in their own languages. This would require replacing the string literals with labels and providing a language file with the strings.

An additional suggestion:
I find it a good idea to translate even some book names.
e.g.
Zion's Harp (Hung.) -> Sion Hárfa
Zion's Harp (Ger.) -> Zionsharfe
