Welcome guys from Europe!
A few years ago, when I joined the TCDB community, I faced the same problem.
I asked for the registration of a lot of teams into the database, which I did as follows:
As the official name of all European teams, I used the name that the UEFA uses, because every European team is a member of UEFA. (https://www.uefa.com/#/)
However, I did not use special letters such as 'ü', 'ö', 'đ', and so on, because the most people cant type special letters like East European letters. In Slavic languages there are 40+ characters, while in English only 26.
If a team's name was already existed in the database, and was incorrect then:
1.) For few data, I requested the modification as above
2.) In case of many data, I used the existing one. The reason for this is that it is easier to rename all at once at the database level.
Example:
Bayern Munchen
The TCDB use this name as 'Bayern Munchen'
This name is incorrect because the team's official name is 'FC Bayern München'. Without special letters: 'FC Bayern Munchen'. In my opinion the last one is the correct.
https://www.uefa.com/teamsandplayers/teams/club=50037/profile/index.html
The situation is similar to most English teams like Arsenal vs FC Arsenal, Chealsea vs FC Chelsea and so on, or Spanish teams like FC Barcelona, or Real Madrid CF.
But be careful. UEFA uses the official name only on the site of the team, on many pages they use only a short name of the team, like ’Bayern’. You have to check this for yourself. If you build a database, it is your responsibility.
Example for a new team:
I requested '1. FSV Mainz 05' because it is the official name of the team by the UEFA, and not just the common used name ’FSV Mainz’ or ’Mainz’.
Why did not I use special letters? It's simple.
If I can not use the Czech, Slovak, Swedish, Norwegian full fonts, why should I use the easy-to-describe "ü"?
So I only used the English character set.
Sorry for my not too good English, I hope it was understandable.