What language would you pick first, if you're going to add non English to London trains?

The problem with UK announcements is that they are piped to multiple places in the station, which is all hard surfaces and produces lots of reverberation and echo. This often makes them hard to understand even for natives. Also there are some stations with really terrible old speakers , such as horn speakers.

I'd ask the airports themselves for the nationalities of the tourists departing through them, and specify whichever secondary language was most relevant for trains likely to be used by tourists accessing those airports accordingly.

If I had to guess, French, German, or Spanish, in that order. But it may well be that e.g. Heathrow has a lot more Arabic, Stansted gets a lot more German, and Gatwick gets a lot more French, Luton gets the Spanish tourists, and City is mostly business trips or something.

You're correct about the acoustics, but foam panels are a thing that can be installed (or not) independently of this.

Arabic?