Demand for a linux app?

It's odd. Opening the same file in Mp3tag in Windows (using the same portable installation) displays a normal ü.
It's also not a general issue. A different test album I tried displays ü, ß, á, é etc. normally.

Hexdump of the composed variant:

hexdump -C 08\ Die\ Sümpfe\ Der\ Traurigkeit.flac | grep Traurigkeit -B 2
000000b0  00 00 44 61 74 65 3d 31  39 38 34 21 00 00 00 54  |..Date=1984!...T|
000000c0  69 74 6c 65 3d 44 69 65  20 53 c3 bc 6d 70 66 65  |itle=Die S..mpfe|
000000d0  20 44 65 72 20 54 72 61  75 72 69 67 6b 65 69 74  | Der Traurigkeit|

Hexdump of the decomposed variant:

hexdump -C 01-01.flac | grep Kunststu -A 1 -B 1
00000050  76 66 36 32 2e 33 2e 31  30 30 19 00 00 00 54 49  |vf62.3.100....TI|
00000060  54 4c 45 3d 4b 75 6e 73  74 73 74 75 cc 88 63 6b  |TLE=Kunststu..ck|
00000070  20 28 4c 69 76 65 29 0d  00 00 00 54 52 41 43 4b  | (Live)....TRACK|

Mp3tag treats the decomposed version differently from the composed one.

To rule out my config I created a clean test installation of Mp3tag beside my configured portable one.

File that is displayed composed + file that is displayed decomposed:

filtering for ü doesn't show the decomposed form

filtering for the decomposed variant doesn't show the composed form

Mp3tag also retains the decomposed variant when saving (I appended 1 to the TITLE and saved the file, ü remains decomposed).

I can send you the problematic file/album if you're interested in investigating this.

Yes, äöü are displayed correctly over here, using v3.34.1 (64-bit, standard install i.e. not portable).

Florian, with a little help from google i changed the Mp3tag.desktop file, and now when i rightclick a folder in double commander, i choose Mp3tag, and Mp3tag opens with al the mp3s in that folder. Here is the desktop file :

[Desktop Entry]
Name=Mp3tag
Exec=sh -c 'WINEPREFIX="/home/yourname/.wine" wine "/home/yourname/.wine/drive_c/Program Files/Mp3tag/Mp3tag.exe" /fp:"$(winepath -w "$1")"' _ %f
Type=Application
StartupNotify=true
Path=/home/yourname/.wine/drive_c/Program Files/Mp3tag
Icon=2FAC_Mp3tag.0
StartupWMClass=mp3tag.exe

Does this only work in double commander? And only with a single folder?

The approach I chose for my Mp3tag on Linux guide uses a separate .desktop file for the context menu as well as a helper bash script that allows calling Mp3tag with single or multiple files/folders, mimicking the behavior on Windows.
This is the section in question.
Does your approach offer advantages over the one I chose?

The additional upside of using a helper script for me is that it allows easy integration of Mp3tag into other programs.

For example I've integrated it into yazi yesterday.
yazi mp3tag
In case there are other yazi users in here (unlikely, which is why I didn't include it in the guide):
in yazi.toml you can add mp3tag as an [opener]:

tag = [
	{ run = "/mnt/mp3tag/mp3tag-context-menu.sh %s", desc = "Tag",      for = "unix", orphan = true },
]

and then assign it to folders and audio files in the [open] section:

rules = [
	# Folder
	{ url = "*/", use = [ "edit", "open", "reveal", "tag", "convert" ] },
...
	# Media
	{ mime = "{audio,video}/*", use = [ "play", "reveal", "tag" ] },

This is for all file managers in linux, i edited the code. It stopped working.

I did not test your finding, i noticed it after i found mine.

So now when you klik on a folder and choose open with mp3tag, it opens the folder with all the subfolders at once

Wow, this is brilliant! I use Yazi and Mp3tag via wine on Gentoo Linux and shall be trying this out. Thank you so much!!

Got mp3tag to work perfectly for me on Linux Mint using bottles. I documented my process here: Mp3tag on Linux Mint (via wine, bottles, flatpak)