We should think about warning users when they want to display content in a non-en-US language
Localized bundles ask users first about the trade-off between getting content displayed in the browser locale or sticking to en-US. But no such warning is happening when users do the respective modifications on about:preferences#content themselves. We could think about adding a warning for that (too).
Reported by Xiaoyin Liu via HackerOne.
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