Windows Explorer doesn't use the COMMENT field for the comments in FLAC, but instead uses DESCRIPTION.
If you'd like Mp3tag to instead transparently write the DESCRIPTION field when you enter something in COMMENT, you can add a dedicated mapping at File → Options → Tags → Mapping using
| Tag: | VorbisComment |
| Source: | DESCRIPTION |
| Target: | COMMENT |
The ID3v2 language setting mentioned in the previous post (now at File → Options → Tags → Advanced) only applies to ID3v2 tags, used, e.g., with MP3 files.