Provide an option to define a language code validation callback

[santhoshtr]

If the application has to bypass or do something special with the selected language code, a callback validation method is required.

For example, to bypass qqq language selection, which is a message documentation artificial language code in Mediawiki

[elad661]

should such callback be called every time the list is built (and on search) so that it won't display invalid languages?

I think having the option to deny user from using a language after they found it on the list is a bit odd.

Either way, I'd be interested in implementing this.