Require language field

[dfabulich]

Right now, you can leave the language field blank. Not that many games do, only 778 out of the ~15K games in our DB.

But we could make it mandatory, instead.

See also:

• Set the language on all of the games that don't list a language #1348

EDIT: I think I want to close this issue and its subissues in favor of:

• Require users to check a box in order to leave the Language field blank #1349