When people sing without knowing or using the text, they often sing as lalalala...
I am not sure why la is used widely as a non-lexical vocable (it could be an arbitrary selection that gained popularity), but I can tell you it is not a worldwide phenomenon.
In Indian Classical music, there is a segment called alap (or alapana) which is non-rhythmic improvisation done without lyrics. Typically, the singer says "aaa..." as he vocalizes (other vowels are used, albeit very rarely). In other variants such as the nomtom (or tanam) use nonsensical syllables such as nom, tom, na, tana, rana, and so on.
In fact, there is whole a class of (rhythmic) compositions called taranas, which are entirely made up of meaningless syllables.