Ethrosian, is the most widespread language in Estreoth, currently spoken by some 60% of the population. It originated in Morith in the seventh century and became a widespread language in the centuries that followed. It is very important to note that Ethrosian is virtually identical to modern English, with only difference being differing words between the two language. Aside from a few words, the languages are so close English speakers from our world can perfectly communicate with native Ethrosian speakers.
Given the nature of Estreoth, it is entirely possible that Ethrosian was introduced by native English speakers. However there is no conclusive evidence.