"The girls are at the beach."
If the plural subject were omitted, then you could not omit the plural suffix. If the subject was a plural, but not of people, then you would not use the plural suffix. Otherwise, same meaning, one is just a shorter way than the other, I think. Check here: https://www.duolingo.com/comment/7738500
I live in north Cyprus and plaj (presumably from the French ) is used for what the English call 'beach' or 'sands'. I do think more atttentio n should be given to regional variations in all languages. I am often confused by Americanism s, eg 'fall' means drop down - 'autumn' is the third season of the year!!