Apparently, to add a new language to Botania, you need to register it in https://github.com/Vazkii/Botania/blob/master/MODSRC/vazkii/botania/client/lib/LibResources.java. 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.

Doesn't forge load all the xx_YY.lang files from assets/modid/lang automagically? You shouldn't have to do ANYTHING on your end, it just works...

Hm, interesting, I was not aware it did that now, thanks for pointing it out.