How to get a source language (English) .po file?

Umm hmm? Every po file contains the English text in msgid part.

Ouch!!

Yes indeed, I can simply convert any (of our) .po files to an English one. The point being that .po comparison works source-language agnostically, so it's easier to make it this way.

I've made a converter script last weekend, using awk. I want to share it with everyone interested. Do we have a place for such things?

If you still have it, dropping a note with its code in mw:Help talk:Extension:Translate/Off-line translation sounds a good idea, mw.o is full of such stuff.