Fixes #1340 ACP login form language

Its impossible to know what is user preferred language for ACP LoginForm before he actualy login there.
however we may assume that he expect to see the same language as he used in frontend.
Therefore we remember language that was used on front end in `mybb[frontendlang]` and then ACP LoginForm can optionally use it to assume language that should be used.
#1340 

This will always set the cookie to the board default. It should set to the user's choice and if that is not set, the board default.