The position is based at LHR but you will still be flying to both CDG and FRA and this is why you need to know either French or German.
A bit of advice: If you don't speak either French or German, don't bother going to the assessment day as it will be a complete waste of time as you need to pass a language test. But, even if you have been invited to FRA and don't speak German but French, you can still do the language test in French.