You get the current language of the install with the function get_locale(). WordPress save the language in theoptions table, option_name is WPLANG. But inside the Multisite install is in the table sitemeta.
But is not important. You should use the default functions to load a language file inside the theme template or the plugin.
- Theme:
load_theme_textdomain() - Plugin:
load_plugin_textdomain()
You find a small example inside this contact form template, public.