You can reference different jquery.timeago.js when having one page per language, but this is static. To do it dynamically what you need to do is reference jquery.timeago.js in your page, at the end of your master or main file (or the page you use this library) you retrieve the object $.timeago and change the field settings according to the flag of your language:
if (typeof($.timeago) != "undefined") {
jQuery.timeago.settings.strings = {
prefixAgo: null,
prefixFromNow: null,
suffixAgo: "il y a",
suffixFromNow: "d'ici",
seconds: "moins d'une minute",
minute: "environ une minute",
minutes: "environ %d minutes",
hour: "environ une heure",
hours: "environ %d heures",
day: "environ un jour",
days: "environ %d jours",
month: "environ un mois",
months: "environ %d mois",
year: "un an",
years: "%d ans",
wordSeparator: " ",
numbers: [] };
};
}