Because Gaelic speakers change their names depending on the language they are using. While it would be rude to take it upon yourself to call Pierre "Peter" in Gaelic this is not the case with almost all names. Oighrig in Gaelic is Effie in English. If we didn't do this then we would be giving learning a false impression of what actually happens. We also accept the Gaelic variants in English. You would almost never hear most Gaelic names in English.