Prevent adv. search form submission on lang error
Prevent adv. search form submission on lang error

If the user entered something invalid in one of the
language dropdowns, it turns red already and this
commit just prevents the form from being submitted.

Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2