If IP and accept-language don't match, why not make a prominent button (in the language they didn't pick) to allow you to quickly change?
"It looks like you are in Japan, but your computer's language is set to English. Which language would you prefer? [English][日本語]"
If IP and accept-language don't match, why not make a prominent button (in the language they didn't pick) to allow you to quickly change?