How did Gothel in Tangled know the special song for the flower?

I'm going to copy the answer from here.

Credit for this answer goes to Valorum.

The official 'Tangled Junior Novel' (issued by Disney Press) specifically recounts how Mother Gothel learned the "Healing Incantation":

One day, an old woman named Mother Gothel was singing softly to herself during one of her walks along a craggy hillside. She looked down and discovered the Golden Flower.

Just as she was about to pluck the flower from the ground, she noticed that it was glowing. As she kept singing, Mother Gothel's brittle voice become strong and clear. Her old bones didn't seem to ache any more. She looked at her shrivelled hand and saw that all her wrinkles were gone. suddenly, she was young again, and her eyes widened with selfish delight!

Right then and there, Mother Gothel decided to leave the flower where it was, so that she could continue to use its power. It was a secret she kept all to herself. For centuries, she lived contentedly, singing to the flower each day, making it glow with the magic that kept her young and beautiful. And because of her covetous protection of the flower, no one else benefited from the blossom's healing gifts.

It seems that it's the singing that's important rather than the actual words, something that's borne out by the fact that the power can be released either by brewing the flower or crying out some magic tears.

The words of the song are not important. All you have to do is sing.