How important is it that I know all of the words for the technical terms when mentoring in my second language?
Words matter. The problem is that the kids need to communicate the ideas to one another and to teachers, etc. They also need a way to integrate their Scratch experience into a larger programming set of ideas that they can carry with them. 

It is good that you are trying to help, but you need to prepare yourself better first. Build a quick-look glossary for yourself that translates the terms in your native language into the one the kids are speaking. Scratch menus and documentation can help you do this, of course. 

As you work, keep a few index cards and a pencil so that you can extend the vocabulary as needed. The glossary itself shouldn't require you to leave the Scratch environment for lookup. Paper is actually good for this - more index cards, perhaps. 

Another trick you can use, perhaps, depending on the age of the kids, is to have them tell you the names of the terms when you stumble. Make a note of their answer (check its accuracy), and work hard not to stumble there again - more index cards. 

Imperative? Probably yes. Or nearly so. But don't lose your enthusiasm for working with kids. 



I'm a bit of an advocate for the use of index cards everywhere, by students and instructors. There is a description of the Hipster PDA in the answer to another question here.