I did it! I made a translator for the Visitors! I'll post it later.
A quick note: most of the actual props with the writing are gibberish. another annoyance is that the propmakers apparently didn't know/care whether the symbols were right side up or sideways. I've even seen several where they reused a prop several times in different settings. The printed pages with visitor writing are cool, but much of the writing on them is upside down. Then again that could, at least in theory, be a feature. The ancient Greeks would either write from left to right or right to left depending entirely on whether they felt like it. A language that is sometimes written upside down intentionally is plausible.
Things I'd love to check out, but probaly will never get the chance: the digital displays used by the visitors comprise a large part of the basis for my translator, apparently the prop makers coded the computers used in the sets to actually display a font with the visitor's writing. It would be majorly cool to get a copy, especially since it's used the same every time.
roadsigns: several episodes have BiG bilingual signs that I can't see quite clearly enough to try translating. I suspect they're gibberish, but I'm not sure. Most of the small signs are.
EDIT: teh link! [link]