the current language is not supported by your mobile

I had the same problem and followed the suggestion to change language (English) to something else and back to English. It worked for about a month and the "not supported...." annoying message has reappeared. Will try again.

unfortunately this approach does not seems to work, al least for me. I tried a few different combination of languages on Edge 1030 and Xiaomi MI9T without success. 
Same issue for my wife using a 520Plus and Sony Xperia.

same problem on Edge 1030 plus and iPhone 11 Pro with Croatian languages . I tried switch to English on Edge and on iPhone but without success. Garmin its last two year one big garbage. Tomorrow send to store and change for new. 

I am in the US using English, using the 1030 with Samsung S10e. It is interesting that it worked fine for the first couple of months, then the language issue suddenly appeared about two months ago. Apparently this issue is been going on for a couple of years on devices other than the 1030. One would think that Garmin would have had a fix for this by now if they cared.  

I had the same problem with my Edge 530 recently. I contacted Garmin who said that it was not a common problem and they didn't have a fix for it. Apparently they are currently looking into the issue. I previously tried changing the language to French and then back to English, which worked for a few months. This time I tried adjusting the auto pause settings on the device. I previously had it set to pause at a set speed. After adjusting this setting to the pause when stopped option I have not had any error notifications. Hopefully that will be the end of it. I really like the 530 as a device, but the Garmin software continues to have frustrating bugs.

Glad you posted this. I experienced the same issue. I'm on an edge 530, Pixel 3, and everything is in English. I haven't had any issues on any rides until I changed the "auto pause below a specific speed" option. There had to be some software bug in there when that setting is used that's causing this.

I'll disable that setting and see if things get better.

I'll have to check what my pause setting is. In any event, this problem is occurring more and more infrequently. It resolves on its own without any fiddling on my part eventually (during the ride, or a ride or two after).