Translating choose-mirror is not trivial

[Ognyan Kulev <ogi@fmi.uni-sofia.bg>]

Hi,

I find it common practice by new translators (like me) that country 
codes in choose-mirror are not translated and just copied.  (It's good 
to use en.po for reference while translating.)  But that's not all.  I 
didn't understand "US[ default..]" until Denis Barbier point me that 
this is the default value for the select.  All this deserves a place in 
doc/translations.txt.  Would someone commit some text in 
doc/translations about these things?

Regards,
ogi