Because all your german GMs are doing is changing translations of the ENGLISH from the ORIGINAL game, IE USAs commentary. You are not adding anything new, just messing with wording...
IF USA changed its wording, then every nation in the world suddenly gets a message that they have to retranslate, and it woudl not be an LA to do it...not sure if even GMs have access...I think it would have to be a BB to change the original ENglish tag.
than why we take english (uk) for adding new lines and keep english (us) as it is :-)
The problem with that strategy is that then we'd have to see all those extra 'u's and an s instead of a z in things like localize all through the site. (Admittedly, localize isn't a word that should be in BB, but still). The easiest solution would be add something like en-US2, which basically would copy all the text from the default en-US locale but then allow for changes in some of it. If it's true that changing the base English texts would require retranslation everywhere, this is the way around that - if it's not true, this wouldn't be necessary of course.