This bug was carried over from the beta:

Spellcheck is broken for system languages on Windows (no suggestions available)
https://steamcommunity.yuanyoumao.com/groups/SteamClientBeta/discussions/0/597415360364355036/

If you are also affected, please comment in that thread with your system specs, details, etc.

The more people raise their voice, the higher the chance it actually gets fixed by Valve.

Also, just to make you aware:

The file for customizing / disabling dictionaries changed from this:
to this:

However you now can also do it graphically:

With the latest update is now available to all users.

That means you can easily customize your spellcheck language (or disable spellcheck) without having to edit any hidden files.

See my guide for further details:
https://steamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3600107201