I saw in Google Translate the "quite" translates as "eithaf", and eitha doesn't appear. Is this a difference between written or more formal Welsh (since Google Translate is based on texts) and spoken or more colloquial Welsh?
The final -f is frequently omitted in informal Welsh writing because it is very often not voiced, or only lightly voiced, when spoken.
Google's Welsh is still pretty flaky - don't rely on it for anything but a general gist. MyMemoryTranslated.net is usually a good source of real-life examples, although it, too, suffers from occasional oddities and vandalism. The examples are very often from a more formal register of Welsh, though, such as the minutes of the Welsh National Assembly - those are reliable, having been done by experienced, professional translators.
Does eitha mean 'slightly' or 'very' (or both)?. It seems that 'quite' is in the process of changing its meaning and actually means different things in different English speaking countries.