I've seen this on several people's accounts, and I want to know what it is. Can anyone tell me what a Language Tier is?
A language tier is a measure of how good you are at translating sentences. This belonged to a feature called Immersion which Duolingo recently removed.
It was a part of the now-removed Immersion feature.... you will be missed, immersion.
Google is good for this sort of question: https://www.google.co.uk/search?q=duolingo translation tier