To edit the disc:

1. Use MagicISO and extract the Mk3's usw_load file
2. Use NavTweak, overwrite the German menu text with your desired language
3. Drag and drop usw_load back into MagicISO
4. Save the ISO image

You can change version number if you want (to higher than 3-1/63, eg 3-1/64), then the OS will load over the top of 3-1/63

Or: simply load an older OS version older than 3-1/63 (eg: 3-1/50), and then load the modified 3-1/63 on top of 3-1/50

Both methods will work
Version number is stored in sw_rel.tbl, edit with a hex editor.