What language does rin and len kagamine speak?

Rin and Len Kagamine, being fictional characters, don't actually speak any real language.

However, they are voiced in Japanese by Asami Shimoda (Rin) and Maaya Sakamoto (Len).

They also sing in Japanese, although their lyrics can be translated into other languages.

So, while they don't have a native tongue, their primary language is Japanese.