Mp3tag looks at the current file. As this is unique, there are no duplicates.
This would require to write driver software for a lot of devices - as they all have their peculiarities I would say it is much easier to rely on the ablities of the devices interface. if this interface cannot deal with a filesystem then it si pointless to ask this feature from mp3tag. if the feature is there - why have it twice?
This would require that you have a language field set properly.
Also, a lot of groups play with the language and deliberately use mis-spelled words (see e.g. P!nk, Prince, Slate)
Did you have a look at the various sections of this forum? E.g. web-sources shows a number of scripts developed by other users.
And also, the forum in general is about sharing knowledge about mp3tag and tagging.