To extract a .po file from valheim run the following
python extract.py -e valheim_out.po <C:\path\to\valheim_Data>
This .po file can then be opened in poedit or simmilar program to do the acctual translation.
When the file has been translated the new translation can be patched in.
python extract.py -p valheim_out.po <C:\path\to\valheim_Data>