In Moroccan and Algerian Berber, as in the Sahel, language names are feminine, and are formed with the feminine circumfix t-...-t: Taqbaylit, Tarifit, Tamazight... In Siwi, however, languages are masculine, as in Egyptian Arabic. Ordinarily, Siwis simply call their language Siwi. When they want to specify the language as opposed to anything else from the oasis, they call it Jlan n Isiwan, "speech of Siwa/Siwis".
If you're writing in a more westerly Berber language, it's quite appropriate to nativise this term into Tasiwit. But if you do so when writing in a Western language, you're just imposing a Moroccan/Algerian convention on a language whose speakers are even less familiar with it than your readers are. On top of that, the feminine of Siwi in Siwi is Tsiwett, not Tasiwit as it would be further west. So just stick with Siwi, OK?