They are just Unicode strings in the vector tiles (which are their own format, nothing to do with SVG). It's this particular demo that is missing basic Arabic support.

  string_value: "شارع المدينة القديمة"

Someone's already made an issue: https://github.com/pnorman/osmf-shortbread-todo/issues/9

Found via what seems to be the announcement of this demo/preview service: https://community.openstreetmap.org/t/vector-tiles-on-osmf-h...