Why double spaces in .json?

subj

If you mean inside descriptions after periods it’s because we’re using a fixed width font in the game, so two spaces after a period is the correct setting for readability.
If you mean for indentation of the json itself, it’s a semi-arbitrary indentation level, though we haven’t been very consistent about it one way or the other.
If you’re talking about something else you’ll need to be more specific.

I was talking about descriptions. May be it was some hidden reason for double spaces.

I agree, there are still a few errors in the general .PO export, altough I’ve always considered double-spaces as a matter of a typist’s style. If you (LazyCat-rus) are thinking about a codewise reason to it all, reconsider the fact the double-spaces are usually found after some punctuation, so you could insert a string, some char or both (%s, %c) regardless of it all.