I’m fairly sure that the problem you are running into is that the settings in the component’s Theme Translations section are per locale, not per admin. This means that if two admin users have selected different locales, they will not be translating the same text string. This can be very confusing when some users have the English locale selected and other users have the English (United States) locale selected.
Another thing that can add to the confusion is the set locale from accept language header setting is enabled on the site. When that setting is enabled, some users may have their locale set to English and other users have their locale set to English (United States) without being aware of it. It seems likely that this is what is happening on your site.
Since the English and the English (United States) locales are so similar, it seems that we should be doing something to prevent this type of issue.