I shouldn't worry unduly about the level of discussion - these technical side debates are one of the charms of Pprune, but can leave the relative outsider a little bemused. That said, we probably owe you a bit of explanation....
Flight testing is not a single homogenous subject. Whilst there is inevitable a relatively straightforward trade of Test Pilot, since they have to be able to handle anything that is asked of the aeroplane and it's testing, a lot of the detailed planning and analysis of flight testing is done by FTEs. Any FTE comes from some particular background - it might be systems, avionics, handling qualities, and we tend to specialise. Usually, Handling Qualities FTEs are closest to the TPs, for the unsurprising reason that handling qualities is the subject that most draws upon a Test Pilots peculiar skills. It also tends to be HQ people running training establishments like ETPS. This has arguably led to a view in much of the FT world that performance and handling qualities (and especially handling qualities) is the be-all and end-all, and that anything else is a "ride along". For light aircraft testing (my own specialism) this is fine, but for bigger more complex aircraft it's not really true any more since the enormous complexity of systems on a modern aircraft like A380 or Typhoon (to pick two at random) means that although P&HQ must be right, so too must a huge raft of systems assessment, which can potentially take far more time and analysis to get right.
On the other topic, I'm not sure how well equipped I am to answer meaningfully on languages, since I haven't worked much outside the English speaking world. Speaking for myself, I didn't elect to do a second language within my degree, on the grounds that opportunities to study languages are far easier to find outside a university than the technical subjects that were also available to me there - but that's just me.
G
N.B. For what it's worth, I know one FTE and one TP who both went to work for Pilatus (in German Switzerland) and didn't actually learn any German until well after they'd got there.