Google Translate analyses translations on documents on the web, so if there aren't many documents available in both of two languages, you can infer that Google Translate won't work well for those two.
It works exceptionally well for Germanic languages, however, which have grammatical structures that closely resemble that of English. Dictionary translations are thus effective.
This is a little off-topic, but I can't find the answer and I realized this might actually be an excellent place to ask this question:
I've noticed Google Translate seems to do a pretty bad job sometimes of Russian-to-English and vice versa, assuming that my human-translated documents are correct. Does anyone know of a better free service for machine translation for Russian?