IF each and every tools, SW-dev and GPS-producer would introduce 'his' terms of use for ressources, services and items, it would be CHAOS total!

That's clearly a no-go. There are allmost all pilots able to fly a plane and transit and even land in this wonderful country without the need of translation tables for these services.  Call them TWR, APPR and so on - as you want and you will be warmly welcome. 

Don't try press rhules on local countries they never add a bit to safety!  Please beware, that from nearly 100'000 humans possesing one or the other license to fly in France there is more then the half NOT proficient in english / aeronautical english! (I guess the part is even over 80%. 

When the FCL standard for language proficiency was introduced in the 2000's, I 've got a long waiting list for a place in the course. 

3 cycles later (I repeat the test each 6 years) I was shocked to see nearly no more pilots with PPL coming. And the test facilities has been drastically reduced due to few candidates. So I was in a re-check session for 5 to 6 with all commercials, prof. heli pilots and two stagiaires as future airspace controllers..

That's the reality. By the way, ULM +PPL makes 60'000 license holder in france..

Keep it going like it is

 

The callsigns are available in both French and English language in the official AIP. Here an example of entries for Chambery (LFLB):

As for the fact that Skydemon is an international app and all other countries have English callsigns, wouldn't be possible to pick the "EN" version for the callsigns in Skydemon, instead of "FR"?

I suspect it's simply that SkyDemon gets its data from.the official French sources, and thar is how they represent it. 

AIUI, SD do not create those data themselves - they reflect the official information (which is how it should be IMO, to reduce opportunity for error).

You can use the English terms in France.

They are even on the charts, see for Toulouse Blagnac (you can call BLAGNAC GROUND, BLAGNAC APPROACH or BLAGNAC TOWER).
As an aside info, French being an officially approved ICAO language it can be used (I'm not saying it's always a good thing).

As a general rule, Skydemon uses English terms for radio stations / callsigns around the globe, which is good and international standard. Schiphol Tower, Frankfurt Ground, Venezia Approach etc. But why is France an exception to this rule? Tour instead of Tower, Sol instead of Ground, Prevol instead of Delivery, Approche instead of Approach etc. Maybe it's only me, but I find this stuff distracting in-flight. And if I call up with "Tour" instead of Tower, ATC might very well reply directly in French.

Of course, even in France, ATC is used to their English names, so there is no reason to call their radio stations in French language, while speaking in English for the rest of the conversation. Maybe it's time to fix this in Skydemon and align French radio stations with the rest of Europe / World?