spoof english + htmlform <details> can leak app language

just throwing this up so as to not lose track

• https://searchfox.org/mozilla-central/source/dom/locales/en-US/chrome/layout/HtmlForm.properties
• so not every thing that is localized is caught by spoof_english, so we have some holes to find and fix (to be safe)
• in this case (details) I don't think I can measure the difference

edit: and here's en-US as the control

cc: @pierov nightly which doesn't have the datepicker patch