If applying in Britain, and a few (obvious) other countries, I would have thought English would be fine-and that speaking another language fluently would not put you in any greater advantage, as far as selection is concerned (
afaik).
If you can speak & write French fluently, you could conceivably give the AF Cadetship a go, likewise with German for Swissair & Lufthansa.
Y para ti Joaquin, recomiendo sino Emirates, BA o posiblemente Iberia-pero que se yo...
Rgds,
AJ