I usually reserve squiggles for inaudible dialogue instead of different languages.
If you're okay with putting a little more effort in but don't want make up a straight-up conlang, I'd either:
- come up with a general 'look' for how the fantasy language looks like when written down (which doesn't have to be intelligible in any way). You might need to do this anyway if you want to display written documents in the fantasy language in your comic later on
or
- write down what the dialogue sounds phonetically (which again, doesn't have to be intelligible, it can be something like 'Ef, ri alkrash breh undulle!', which doesn't mean anything in particular; I just strung some random syllables together XD)
Otherwise, if you just want to get it over with and put in minimal effort, I second 'google translate it to a little used language that you target audience won't know' :]