First of all, “desires” are two kinds:
- Desire for sense pleasures. This also has a wide range from kamaccandha (removed at Sotapanna stage) to kama raga (removed at Anagami stage) to rupa raga and arupa raga (removed at Arahant stage).
- Desire to attain Nibbana. This is chanda iddhipada, one of satara iddhipada or four bases of mental power: “The Four Bases of Mental Power (Satara Iddhipada)“.
Anything that one “desires” to do in order to attain Nibbana falls under chanda iddhipada.
Desire to learn Sinhala is of the second kind. And it makes sense because Sinhala is the closest to Pali. It is easier to learn Pali, if one knows Sinhala language.
– On the contrary, Sanskrit (which many people try to learn) is the wrong language to learn. Sanskrit words are deliberately composed to provide mundane meanings and also to make enticing and appear sophisticated. The Sanskrit word for paticca samuppada, pratityasamutpada, is a good example. It sounds sophisticated, but does not convey anything.
- The Buddha specifically instructed not to translate Tipitaka to Sanskrit, because of this problem; see #5 of “Preservation of the Dhamma“.
The best way to learn Sinhala could be just go to Sri Lanka and spend a few months. I am not sure whether that is practical. Another possibility is to learn from a Sinhala-speaking person if one is available for regular interaction (i.e, if such a person lives close by).