Ukrainian, Portuguese, Catalan and Dutch translations are added.

Hi,
We are using WTForms in one of our projects at Paylogic. This is a Dutch company and we found that there are no dutch translations at all. We have also colleagues that speak some other languages, so here you go.

In Ukrainian I still need to figure out the plural forms for numbers. I think the formula is not correct because there's a difference between 1, 2-3-4, 5-6-7-8-9-10 in plural form.

Since I can see there are issues with the Russian translations: some of the messages are wrong and there are new messages which are missing, some of them are old and no longer in the new .po files.
I think I can try fixing Russian and plural in another PR.

Please have a look.