The meta question here gives good reasons why tags should be in English, but the discussion does not cover works that were in a foreign language to begin with. I think this is an edge case that needs to be clarified.
If someone wants to ask a question about such a work, should the corresponding tag be the original title, or an English translation of that title? What if it's a bad translation into English? What if there are several? Which one should be used?
This was triggered by my attempt to tag my latest question about a French TV show with its original title. I needed to vent a little on chat when the grave accent è in the tag name was converted to a plain e. @Jenayah referred me to the meta question linked at the top. I wasn't really sure that covered this case.
An extreme case would be the weird 1991 Soviet adaptation of The Lord of The Rings, Хранители (transliterates as Khraniteli). Google Translate renders it as "Keepers", Wikipedia as "Keepers of the Ring", but I was told it translated directly to "Guardians". I can't imagine any other tag than khraniteli. Anything else would detract from its "specialness".
I think that rather than a "tag in English", we might have to go for "tag the best-known title".