(And update the en-US version with some strings that we don't use but
might if we updated to a slightly newer version of the updater before
they got rid of the dedicated update window)
From some 2017-era langpacks I had downloaded. These strings aren't in
current langpacks, which use a different update mechanism, but we're
bundling a modified version of updates.xhtml from Firefox.
It was previously set from general.useragent.locale, but that's not used
by default, meaning that date interpretation and some other things might
have been incorrect for everyone who didn't set a locale explicitly and
toggle matchOS...
Remove localizations in standalone.dtd and replace with editMenuOverlay.dtd
files from the Mozilla language packs
Also remove some other unused strings
