Airlines, including EZY, still value additional languages. Actually rostering them onto relavant flights however is just not feasible. As someone has already mentioned, 4 sector days often take in different countries plus there is additional cost in rostering flights that way in that individual crew members become inflexible.
Also, despite nightstops being few and far between at most EZY bases there is still often a preference on the types of routes crew fly and this is usually, but not exclusively, associated with how lucrative they are both in sector pay and commission. As a result some crew would be left with routes they didn't necassarilly want whilst also potentially being out of pocket.
That said there are plenty of foreign speakers right across the UK network who i'm sure probably see fairly frequent service to destinations where their language can be put to good use whilst those crew based in CDG, ORY, LYS, MAD, BSL, GVA, SXF, MXP, FCO and soon LIS, do have to speak English plus the language of their home base.