I thought "Yes" was "Kessa", and not "Issa" . At least that's what it says everywhere on the internet.
The course is not made by the inventor, but in an official page of high valyrian does say that yes is kessa, and that one is made by the inventor
According to this page, both are forms of the verb 'to be', but issa is present tense, whereas kessa is future tense, so a precise translation might be "it is so" or "it will be so" respectively. So perhaps you'd use "issa" in answer to an question about how things are, but "kessa" in response to a request to do something?