Flags are not languages (there's a whole blog about that)
It just causes some issues, such as some languages having multiple flags, and some flags representing multiple languages. Then even if you pick a regional flag for that language, it's not certain people will understand what language it is. So it's better to just write out the name instead.
This applies to everywhere on this forum, even in the translation posts. It sure does look neat with the flags, but it does not help when it comes to duplicate flags and obscure flags.
Mandarin Chinese [Traditional],
Minnan Chinese [Traditional]