As you might know, for a smart form, you can check translations avilable in SE63 transaction.
In menu Translation > ABAP Objects > Other long texts, selecting Forms and styles (FS) > SAP Smart Form (SSF). From here, you can enter the name of the smart form in the field Object name and entering source language and target language.
The solution, however, is more "check if the smart form is translated to a language" than to "get all languages to which smart form is translated".
Maybe better than this, if you want to know available translation for a smartform, use table STXFTXT. Filtering by the smart form name, you will get text entries in the languages they are. So, in fact, if you sort by language and delete duplicates you will get available translation of the smart form (although you can have partial translations of course).