I am working on bug 1846224 and I am planning to implement the new language spoofing feature in a way that makes is possible to not just spoof en-US, but any locale string. Like Zibi I don't think we need to change GetRegionalPrefsLocales, while it's currently used for the JavaScript locale, we can either switch that caller to webExposedLocales or just use always use the newly introduced locale spoofing.
Bug 1746668 Comment 24 Edit History
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I am working on bug 1846224 and I am planning to implement the new language spoofing feature in a way that makes is possible to not just spoof en-US, but any locale string. Like Zibi I don't think we need to change GetRegionalPrefsLocales, while it's currently used for the JavaScript locale, we can either switch that caller to webExposedLocales or just always use the newly introduced locale spoofing.