Hi,
as written in
this topic, there is a tool that might helps you. You might also want to contact your language "lead" translator, if anyway, or apply to become one (to prevent working on the same files and doing "redundant" work). Anyway, files are located in /core/lexicon/en/ for the original core ones.
/setup/lang/en/ might also be translated.
Hope that will lead you to the right start
Cheers
☆ A M B ☆