Hi,
Sorry in advance if I over explain myself but I wan’t to be sure we are in same page. 
To translate the last OJS version (3.5), the recommended way is using weblate.
The tool will assist you in the process and coordinate with other translators.
After this Alec manually packages all the OJS releases including the new language improvements.
Here you have more information about the process:
This process have some benefits, but also some downsides:
You all the support weblate offers during translation… but you are kind of “blind translating” because you can’t see the context of the strings or test how they look like in the app.
Weblate is the usual-recommended workflow… but there is also an alternative: You can commit your lang files directly to OJS repo. Committing to repo is, by the way, the only option if you like to contribute to older OJS branches.
If you want to work without weblate, first step is be sure you are coordinated with the person or team that is maintaining the translation. If you are not, your work will collide with your fellows one.
Yes, for “non-weblate” translations, if you are working with “docker volumes for locales” will be also my election. What it’s not clear to me is what do you mean by “use the UI to update” as in 3.5 (if I recall well it was removed in 2.x) there is no translation plugin any more.
So, the process will be: Set your environment > Edit your lang files > Commit to your OJS fork > PR to PKP repo.
Even inside docker, I tried to remove and install the locale. It didn’t complain, so something should have been done. Nevertheless, nothing changed. I am not sure if OJS fetchs it from the web or just uses whatever is has local.
As far as I know, when you remove a locale from the webUI, the files are NOT deleted but lang is disabled. But when you install from the webUI, the files are downloaded from OJS repos again and updated.
Notice the downloaded version will be the last packaged, but could be unsync with the one in weblate, so this is why contacting with lang coordinator is so important and weblate is the recommened way.
About your issue… probably a cache is interfering here… Did you try cleaning OJS and your local caches?
PS: A mention to @asmecher to confirm he agrees in the working proposal.