The native tongue to the Tovag Domain is Ancient Flan, but it isn't available in-game. The Greyhawk version of Flan is however, though it is stated that they are nothing alike. Would be nice if Ancient Flan was made available in-game.
The language system supports that we make our own acronyms for languages that may not be listed. The list is not an exhaustive list of supported languages. You are not restricted only to what's in it. You can therefore go ahead and make your own acronym for the language, I would suggest "AFl" if it is not already used.
The main reason
Ancient Flan is not listed is simply because at the time of the language system's inception, the Burning Peak cluster was thought lost and destroyed. We reviewed this decision in light of the release of 5th edition Van Richten's Guide and made Tovag a valid domain of origin again.
//Addendum, we do add new languages to the list if we find we're missing some. Ancient Flan will soon be on the list. Keep them coming if you find any missing one from canon D&D sources.