I've moved your post to a dedicated topic — it seems to be a separate issue.
Mp3tag reads Unicode characters as long as they are encoded in Unicode (UTF-8 or UTF-16 LE/BE). However, some tags are stored in the system codepage of the locale, which makes it difficult to impossible to read.
Are you using Mp3tag on Windows or are you using the Wine-based macOS version? Could you provide an example file which shows this behavior (preferable via private email)?
Kind regards
– Florian