Music in itself has no language. It is purely rhythm, meter, scale, notes etc. Lyrics can easily be substituted with any syllables. Music can be enoyed for the music without the lyrics. In fact remove the lyrics and you have a nice accoustic version. So many local and international groups have transcended the language barrier in the music industry.
Lyrics add an extra dimension to the music. Lyrics can be used in different means - in rap lyrics and the lyrical rhythm is of utmost importance even if its nonsesnical. In techno, trance and house the lyric is usually the same set of words repeated over and over again, being a beat centric genre the lyrics are selected to fit the tempo.In death metal the lyrics are absolutely distorted and harsh. A lot of genres of music can be enjoyed without lyrics at all.
On the other hand lyrics can add a whole extra layer of meaning, emotion and feel to a song. The piece of music now becomes a complete song with accompanying vocals, each complementing each other. This is where performance counts big time. An ordinary singer will merely sing a song. Performance can be as limited to varied facial expressions and hand gestures, a body language that can emit the feel or it can be as exhubarent as moving around, bringing in props and portaying it like a story.
However, the fact remains that without knowing the lyrics it is hard to judge if a contestant is giving the right feel of the song. But conveying the feel of the song is more in performance and less in singing. Multilingual singing contests judge voice quality, throw of the voice, pitch, scale, various mannerisms. I would expect the jduges familiarize themselves with the original song first to know the basics like rhythm etc.
But when you consider music with no boundaries you have to look towards the Eurovision Song Contest perhaps the biggest and longest running reality music contest - over 40 countries participating, 600 million viewers in over 140 countries around the world. All kinds of music from Nordic, Greek, Gaelic, from Western to Middle Eastern, From Classical and Folk to Pop and Rock. Winners voted by the viewers of each of the participating countries. It really shows music has no language at all.