New feature: allow loading other language files for reference

[mikkorantalainen]

While translating a program to e.g. French, it is pretty common that the translator knows multiple languages, e.g. English and Spanish. Virtaal should allow loading en.po and es.po while editing fr.po to allow viewing English and Spanish translations while writing French localization. This would increase the odds of getting correct message on the first try.

[SimonHeimberg]

Pootle can do this: http://docs.translatehouse.org/projects/pootle/en/latest/features/alternative_source_language.html :+1: