In practice we've not accepted PRs with "manual" changes of any non `en-US` locales for many years, however some (older) README/FAQ entries can perhaps be seen as actually inviting such PRs.
Now that l10n updates are running automatically via GitHub Actions, see PR 20749, we're definitely not going to accept any "manual" translation patches so it cannot hurt to de-emphasize localization in various README files.
(Also, since all relevant Fluent files have license headers it doesn't seem meaningful to mention that in the l10n README.)
With the removal of the (standalone) Firefox building code in PR 9566 (a year and a half ago), these files are now completely unused.
Hence it doesn't really make sense to keep building them as part of `gulp locale`, and the existing files in the `l10n` folder can also be removed (thanks to version control, they're easy enough to restore should the need ever arise).
