Steps to reproduce:
1. Go to instance settings/Miscellanious , and move English(United Kingdom) from available to current. Save.
2. Set the default language to English(United Kingdom), then leave only English(United States) and English(United Kingdom) in the Current tab. Save.
3. Create a new blank site with the default options (so it will inherit the instance language settings.)
4. In this site, create a new data definition under Content/Dynamic Data Lists. (Its default language will be English(United Kingdom). Don't add a translation.
5. Create a new dynamic data list based on this definition.
6. Step into the list and add a record to it.
7. Go back to instance settings and move English(United States) from Current to Available tab. Save.
8. Go back and try to add a new record to that list.
Result: It fails with "Your request failed to complete" popup on the UI, without any other details. Also, no errors on Tomcat console.
You can also observe that you can see and select the en_US locale whiile adding the record, even though this locale has been removed from Instance settings (and thus also from this site, as the site inherits the instance's locale settings)
Also, if you switch the locale to en_GB, it quickly bounces back to en_US and the saving of the record fails just as well.
Probably expected: en_US should not be shown as an option here, and the saving of the record should succeed.
Master: at step 8, now only icon for en_GB locale is shown as an option, but the saving of the record still fails in the same way