To me, this would read more clearly if it weren't split out quite so much (makes it seem to me like you're going to do more with the results of this jQuery search. Perhaps:

`if ($('#id_default_language option[selected]').length == 0) {`

But actually (would need to check this), should work to just say:
`if (!$("#id_default_language").val()) {`
(this then matches the setting being done inside the if block nicely)