I am using the language customization, and thank you so much for asking! I am doing a test run with the quiz in English on the Spanish Lessons just because we currently have very few if any Spanish users. Not my typical but it just makes it easier for now.
I had a meeting with my clients and we all had some difficulty understanding what each link and button was supposed to do. My client did most of the quiz and wanted to see what would happen if she went back to the lesson to relearn something, and was surprised by the error message saying her quiz was not yet saved and she would lose all progress if it was not saved. It is confusing, as there is no obvious indication that I am submitting it, that I may have another chance to review it, etc. I had to click on each to see where they took me, and was surprised by what they each did. I'm sure that for most courses this isn't as big of an issue because the student are used to it, but it will be for our users as this may be the only time they ever use this interface.
I find it very strange that these are not customizable? Am I right to understand that there isn't an easy way to change them?
Below are screen caps of what they will say (I hope), and ideally as you pointed out, if they change the language to Espanol the buttons will change as well. The only way I have found to do it is 1- Change it in the script, which means needing to change it again each time I update Moodle (right?) or 2 - change them in CSS (easy enough to hide the text, and give it a background image that is the new button as an image, but then the Spanish will be in English.