Apparently, to add a new language to BigReactors, you need to register it in https://github.com/erogenousbeef/BigReactors/blob/master/erogenousbeef/bigreactors/common/BigReactors.java#L84. This makes it impossible for Resource Packs to add custom languages. I hope this mod will get dynamic localization handling so it is Resource Pack compatible.

Agreed. I also want to redo the UI to use localized strings. I'll do this in an upcoming 0.3 release.

I'm happy to state that this is a native feature in 1.7.10, and will therefore automatically be supported in 0.4. :)