Translation:The rain in Spain stays mainly in the plain.
I realize that the correct translation of 'dans la plaine' is 'in the plain'. But, 'The rain in Spain stays mostly on the plain.' is a saying in English and should be accepted as a correct translation.
'The rain in Spain stays mostly on the plain.' is a saying in English
This is from the film "Pygmalion" (1938) or the play/film "My Fair Lady" (1964). And the full phrase is "The rain in Spain stays mainly in the plain". Video here: https://www.youtube.com/watch?v=uVmU3iANbgk and more info here: https://en.wikipedia.org/wiki/The_Rain_in_Spain
In any case, even though it's not the exact lyric, your translation is fine and has been added.