The Available Locales Plugin allows admins to limit the locales shown in the Interface language dropdown in User settings. The plugin has an Admin settings option, available interface locales, to list available locales for users. It’s enabled once the allow user locale setting is enabled. By default, all Discourse languages are enabled.
eatcodetravel shares the plugin’s repo and notes that it might be outdated, recommending angus’s Multilingual Plugin instead, which also restricts available locales. However, nexo prefers this plugin’s feature of displaying the locale selection within User settings.
1c7 successfully tested the plugin, and eextra inquired about its compatibility with anonymous users’ locale settings, to which eatcodetravel replied that the feature might not be supported, suggesting angus’s plugin for possible support.