upgrade site to have ENGLISH button

Good morning I am an English translator and go into businesses every day to translate documents letters phamphlets etc. alot of times I am asked if i can add- on an English button onto their webpage to show all their site in English also. I do not know how to do this and need help. I need a good price and I need it to be done within 4 hours of contact i could supply the english translation if necessary but would rather it be done by you and then verified by me (Iam British) the original sites will be in Brazilian Portuguese. Thank you. Paul