When a page is marked for translation with a priority language set and the option - Prevent translations to languages other than the priority languages set, translators are still allowed to translate a translation unit and then click on the save button. After clicking on the save button, the translators are moved to the next message but the translation is not actually saved due to the Prevent translations to languages other ... option being set.
The translator can see the error by reopening the previous translation unit.
See below images,
After discussing with @Nikerabbit, we've decided to disable the save button and if possible show the warning at the beginning, when the translator opens the translation unit for translating purpose.
As per discussion in T217046