I have managed to reproduce the issue with the steps provided in Comment 15 on the following versions of Firefox with RO locale on Windows 10 Pro 64-bit and macOS High Sierra 10.13.6:
The corresponding Chinese (zh-CN) language pack has been used to reproduce the issue, since it is easier to distinguish the changes occurring in the browser upon installing it, as stated in Comment 15.
The following were observed:
After step 5 some of the pages are a mix of the RO and CH languages. Proceeding with step 6 and updating the language pack will cause most of the windows to revert to the initial language (with CH still present in the Recommendations sections of the both the Recommendations and Extensions tabs). Refreshing the page will cause about:addons to become mixed again, to a higher degree.
Restarting the browser will fully localize in the installed language pack (CH).
After installing the language pack, a new tab appears in abbout:addons (languages tab, where the installed language pack can be seen). Disabling the language pack from there will cause the pages to jumble up again. Restarting will properly localize in RO.
After restart, enabling the language pack again from the languages tab will cause the issue once more which is solved after another restart.
In conclusion, I did not manage to make the issue persist after a browser restart.
For ESR and Nightly, which do not have an option to change the browser language, the corresponding language pack has been installed and the language has been changed via the ‘ intl.locale.requested’ pref, set to ‘zh-CN, zh’ as the used language pack was the Chinese one.
I’ve also tried a similar method of reproducing, more similar to the scenario of the reporter, i.e. having FX67 RO locale installed, going to about:preferences and changing to an alternative language (en-US), restarting the browser (which caused the update to the latest version) and then noticing the mix of languages in about:addons.