Question

So I’ve been able to figure out how to use gettext to translate for a certain language, setting the setlocale, how can I figure out what user probably want for language to be set?

Answer 1

In this order:

1. Ask him (for example as preference setting and/or using a landing page).
   Remember the user’s choice.
2. Parse the HTTP Accept-Language request header ($_SERVER['HTTP_ACCEPT_LANGUAGE']).
3. Default to English (or whatever else is appropriate as default for you).

Pick the first option that makes sense for you. Always allow the user to switch to another language easily. Usually through links in the page header or footer.

The URL should contain the language choice. I.e. ultimately the current URL is the deciding factor for the language, the above options just redirect the user to the appropriate URL. You can use domains/subdomains for this (example.com, example.de, example.jp, en.example.com, de.example.com, ja.example.com) or make it part of the path (example.com/en, example.com/de, example.com/ja).

A forth option is to geolocate the user based on his IP, but I would really discourage that, since location doesn’t have much to do with language.