The French langpack is missing `browser/localization/de/browser/preferences/addEngine.ftl`, I expect that's causing preferences to fall back to English entirely. https://searchfox.org/mozilla-central/source/browser/components/preferences/preferences.xhtml#41
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The French langpack is missing `browser/localization/fr/browser/preferences/addEngine.ftl`, I expect that's causing preferences to fall back to English entirely. https://searchfox.org/mozilla-central/source/browser/components/preferences/preferences.xhtml#41