There are “servers” on Matrix. They are called communities

Here is the relevant part of the documentation for that: https://matrix.org/docs/communities/getting-started/

I’ve never actually used them but in 5 minutes of googling I found ulule. They are based in France and seem to fulfill your requirements.

There is also wemakeit, a Swiss platform targeted specifically at nonprofits. I’ve never used them either, so I can’t tell you how well they work.

The selection prompt exist. It’s just implemented in a way that’s less than ideal. On first start a popup appears informing you about the existence of other UI layouts.

There are just two problems with it:

1. you can’t actually set the UI in the pop-up. Instead you have to click a link in the pop-up which takes you to the corresponding settings page.
2. the prompt shows up after the UI has already loaded. So it doesn’t read as helpful at first glance. To most people, it seems to be an annoying pop-up that keeps you from accessing the app, that you can already clearly see in the background, instead. So they close it without reading it.