Load custom language file in template

Hi, guys!

I’m working on a project right now that needs 3 different language support and i don’t want to use any kind of plugins that switch and add languages.

I have downloaded all three language .mo and .po files and would like to serve them according to a page where user is located (using some simple if) but what i don’t know is how to tell wordpress code to use this or that language file.

I’m thinking about something like this:

if is_page('Russian'){
    define ('WPLANG', 'rus');
}
esle if is_page('Spanish'){
    define ('WPLANG', 'spain');
}
else ...

How to accomplish this, maybe someone knows and has had some experience previously?

Thanks in advance!