Make builder scripts language agnostic

Hi, we are considering start to translate the TRM from French/English to Spanish, I saw there are few .sh directly to build pdf, html and epub for those language. Could you create another one for "es_ES" or make the previous script language agnostic and just pass the language as a shell parameter?

Btw, is it better to translate the book in Transifex or the .po files offline? Thanks!