Are äöü (and quite likely other non-ascii characters) displayed correctly for you? I'm currently in the process of migrating my existing Mp3tag installation to a portable one stored on my NAS, mounted via NFS and executed via wine under KDE on CachyOS. I'm also documenting all the steps since I want to post a guide once I'm done and satisfied.
With this .desktop file Mp3tag opens fine.
#!/usr/bin/env xdg-open
#launch_mp3tag.desktop
[Desktop Entry]
Name=Mp3tag
GenericName=Tag Editor
Exec=env WINEPREFIX="/home/malte/.mp3tag" wine "/mnt/mp3tag/Mp3tag/Mp3tag.exe"
Type=Application
StartupNotify=true
Dragging and dropping folders or songs into it also works well.
But for the first example album I tested, ü is displayed as

Funnily enough, when I select both the u and the square and post them to the forum, they turn into ü again. Copying only the square yields ̈
Extended Tags of the example file:
metaflac in the command line shows the tags correctly:
Mp3tag is capable of displaying äöü. Some of my actions contain those characters and they are displayed correctly:

Is this only a display issue or did I fudge something up?
I've installed the portable version via:
wine mp3tag-v3.34.1-x64-setup.exe to the mounted NFS share, then saved my old config in Windows and extracted it to the new portable installation in Windows (mounted via SMB). I also performed the changes needed for dark mode as described below.
My markdown notes, WIP
Installing locally via Packet Manager on Arch
Community Package (unofficial)
Install via paru -S mp3tag, octopi, yay etc.
Portable installation on Linux via Wine
Optional Prerequisite:
Store Mp3tag on a NAS share
This is useful if you intend to access the same Mp3tag instance from multiple hosts in the same network.
I mount an NFS share from my NAS to /mnt/mp3tag/ on my host system on boot via fstab.
My /etc/fstab contains:
192.168.0.1:/mnt/tank4/Mp3tag /mnt/mp3tag nfs defaults,nofail 0 0
Setting up the Wine Prefix
Create and configure a new Wine prefix for Mp3tag (or use the default prefix if you prefer):
WINEPREFIX=~/.mp3tag/ winecfg
Dark Mode
If you want to use the Dark Mode in Mp3tag, you have change the wine config:
WINEPREFIX=~/.mp3tag/ winecfg
Set Desktop → Interface Appearance to No Theme
It might also be a good idea to change the screen resolution in the Graphics tab if you don't want to scale within Mp3tag.
If you like Breeze Dark as the theme, you have to perform an additional step described here
Importing your existing config from Windows
- In Windows, in Mp3tag, use
File → Save Configuration...
- Still in Windows, export the resulting .zip file in the base of your portable Mp3tag Installation
You could also extract the archive within Wine, but that would require setting up an archive tool like 7z.
Extracting the archive on your linux host might not work as the internal paths use \ instead of /, yielding for example a file called \data\actions\Case conversion.mta instead of the folder data, the subfolder actions and the file Case conversion.mta.