Application::resolveActiveLanguage.php

<?php
/**
 * Returns the active language.
 * @return string the locale.
 */
public function resolveActiveLanguage()
{
    $user = user();
    $matches = array();
    if ($user->hasState('__locale')) {
        return $user->getState('__locale');
    } else if (preg_match(
            '/^\/([a-z]{2}(?:_[a-z]{2})?)\//i',
            substr($this->request->url, strlen($this->baseUrl)),
            $matches
        ) !== false && isset($matches[1], $this->languages[$matches[1]])
    ) {
        return $matches[1];
    } else {
        return $this->defaultLanguage;
    }
}