I have been testing Scrivener 2.9.xx for a while using the Spanish locale and I have customized the main toolbar adding some new buttons. On the other hand, the Spanish locale for beta versions is very incomplete, so I have decided to switch to the original English locale to keep using the program. Then I have realized that some of the customized toolbar buttons disappear. If I change back to the Spanish locale, then I can see those buttons again.
What happens if you make toolbar customizations while using the English locale (different than the customizations you made under the Spanish locale), switch back to Spanish, then switch back to English?
If the Qt framework is saving those customizations on a per-locale basis, that may not be a bug, it may just be how their framework works.
The customizations made while using the English locale won’t stand in the Spanish locale but come back if I switch back to English. I could be, as you say, that customizations are saved on a per-locale basis. I have tried my 1.9 version and it’s curious because the main toolbar seems per-locale based, but the format toolbar keeps customizations between locales, so I couldn’t tell.