Is it anybody from any country?
For local boards, obviously you have to be able to understand the language and for english board same criteria apply. So yes anybody from any country.
Is there any rank range?
Is it by lenght of participation period?
Most be a well established member for english board although that doesn't count for local board.
What are the criteria for being a BitcoinTalk forum, admin or moderator?
In general, it boils down to theymos's decision as back then, you just have to
Report a lot of posts and be a very active member of the community and theymos may ask you eventually.
but the requirements now might be a little bit high since we have members that meet those criteria above in their numbers.