Is it 說 /说 / shuō or 講 / 讲 / jiǎng for "to speak a language"?
The two are interchangeable in this particular context.

The two has different usage in other contexts.

E.g.:


讲 can mean to say in 讲话, to explain in 讲解, to bargain in 讲价.
说 can mean to persuade in 劝说, and it can be a noun in 学说(theory).


The choice of syllables for your situation I would say is based on statistical usage of each. In certain areas 说 is more naturally used because it's used more by the community. And the same is true for 讲.