Weblate 2.16 is almost ready (I expect no further code changes), so it's really great time to contribute to it's translations! Weblate 2.16 will be probably released during my presence at DebConf 17.
As you might expect, Weblate is translated using Weblate, so the contributions should be really easy. In case there is something unclear, you can look into Weblate documentation.
I'd especially like to see improvements in the Italian translation which was one of the first in Weblate beginnings, but hasn't received much love in past years.