Tokio Hotel have some albums in German and some albums in English and some people apparently needed to note that. Also I imagine Korean or Japanese fans of pop music being excited when someone in their countries records something in English, that's so "exotic".
Everything should be tagged for the languages it's sung in. For example, what if a song has lyrics in three languages (say, English, Italian and Azerbaijani) and additionally, a part is scatted? The song should, then, be tagged with the genre it is and furthermore with [tag]english[/tag], [tag]italian[/tag], [tag]azerbaijani[/tag] and [tag]scat[/tag].