Happy to help Mark!
I wasn't so sure what kind of help you're looking for, but I decided to give Crowdin a shot and translated two plugins to Brazilian Portuguese.
The tool is very easy to learn and use. You can compare it with a web tool developed by Debian Linux developers, but the interface is cleaner and provides suggestions, that while were not always useful in the sense you could just choose from and translation is done, but actually helpful to not get stuck.
It also provides a vote system, but I couldn't obviously get into that.
So, yes, it's appropriate for end users that are not acquainted with a shell, IDE and Git. On the other hand, I guess a person that uses Jenkins should be acquainted with such tools, so the lack of context (there is a fancy interface between the translation and what Jenkins actually is) might be a problem.
Another point, I couldn't test how the tool would behave if two people try to translate the same thing at the same time, something that I have the impression that the Debian web tool doesn't do properly.
Since Crowndin seems to be proprietary, that might be an issue, especially if costs would come up sometime in the future (currently, I guess that there is no cost associated) or the tool just ceases to exist.
Finally, not sure how those files would be integrated back to Jenkins' Github repository, that might be a problem (or not).
Hope that helps.
Regards,
Alceu