Language selection guidelines

https://wikia-inc.atlassian.net/browse/AR-18

The question of which languages do we support and what is the criteria for evaluating new languages has come up several times. This is an attempt to create a guideline that covers these topics and guide the decision making process.

/cc @pchojnacki @nmonterroso 
@SebastianMarzjan I clarified this point in 12627523.