I know about Asian watches problem: Cyrylic language interface is not compatible for Asian version of Fenix watches (encoding problems). Any chance to force Russian installation interface for Fenix 5S?
Just a thought. Assuming that all fenix units are the same just with different SW (based on serial numbers) you might be able to clean install an European version.
Although, I'd definitely consult that with Garmin support first, otherwise you might brick your watch. There might be other limitations like unit ID, etc.
In this case, Garmin should print on box - "this version is not compatible with Eastern EU countries" We have Fenix 5, 5S - only English language is available. It's not possible to add the Russian language. :-(