How can I change the language of my mobile Android Connect App without changing system language?

On Android, an application following the guidelines allways should use the same language as is set in the OS (if it is available). If you have multiple languages set in Android system settings, and the first language is not available in the application resources, the application will try the next one. And if none of the list is available in the application, then the application will show up in its default language, which is currently quite commonly English.

If I understood you well, you want that GC Mobile shows up in the language C, while your Android is set to language B. Per-application language setting is currently not directly supported by Android, but you could use a 3rd party solution. I tried googling it, and found for example this one (no recomendation since I do not know it, and there may be more such solutions available): https://android.gadgethacks.com/how-to/change-androids-language-locale-per-app-basis-0172900/

thanks. 

your suggestion requires a rooted device. I'm looking for a solution without root. 

Thats a pitty that guidlines do not talk about providing an individual per app button. 

You can also try this one: https://forum.xda-developers.com/showthread.php?t=1174443

Or just use Google. That's exactly what I did when looking up both solutions for you.

would it be possible to include this in th connect app?

Technically yes, thought they would need to bypass the standard localisation routines of Android. However, you are asking on the wrong place. You need to pose them the question directly, not asking the community on the user forum. In reality I doubt strongly they would ever consider spending their time on a cosmetic feature needed by one in million users. They have plenty of more important tasks and requests on the to-do list, concerning the majority of users.

BTW, I am using also multilingual environment, but do not quite understand why you mind so much having the same language on Garmin Connect as on your Android. 

ok. thanks. I tried it to ask, but the communcation system hangs up yesterday, so I asked here. 

I'm using my systems normally in english, which is easier to ask or search, and also for development. 
But I want my exercises in german, because I'm used to the german names. My phone has spanish as language, because I'm (due to a random event) used to it and want harden my spanish skills. But I don't want my exercise names to be in spanish or in english. 

... another solution would be getting a second cheap phone to use just for GC in German, while keeping your principal phone in Spanish  The added value of such solution is that you would not risk damaging your main phone during sport activities. At most operators you get a spare SIM card for second/backup phone without any problems. That's a solution for couple of euros, while you are asking Garmin to implement a solution (used by virtually nobody) that would cost them many many thousands in development, testing, documentation & translation, training and support costs.

ok. i'll try another macro program. 

I have thought there would be a more native solution. but as I read here there isn't one. 

Thank you for your help.