I don't recall any guest language-specific problems, but I suppose it wouldn't hurt to ask. I hoped the 64-bit/32-bit was hinted at by the example of "Ubuntu 7.04 64-bit", but also added it.

In general, I assume English and 32-bit unless the user says something that hints otherwise. I think explicitly stating default assumptions like this is a bad idea, though, since as you note it may encourage users to omit details.