This reverts commit 17d3ce6 and bits of
previous commits.

We *want* to fail with an error if localized strings are requested for
non-languages, so providing generic English stand-ins seems like
a counter-productive move.

Additionally for anybody starting a new localization it would be better
to start with a language similar to their target because implementation
details might differ. With simple key/value lookups it might seem like
any language could be a template for any other, but Fluent is much more
flexible than that.

While lovingly hand crafted, they don't reflect the structure of how
we're doing to use keys as a namespace and they are getting in the way
of automatically reprocessing these files.

Also we don't really want to include commented keys for untranslated
terms because that is one more thing to maintain and get out of sync
without necessarily providing a benefit. We want people to reference
similar languages when translating.

```console
sed -i -e '/^#/d' *.ftl
sed -i -e '/^$/d' *.ftl
```

```console
sed -i -e '/Rerun SILE/d' *.ftl
git checkout -- i18n/en.ftl
```

In the future on an as-needed basis these could be converted to terms,
but so far there is no use case and doing this should be a translators
implementation choice not something SILE suggests.

Prepend something to all files, then:

```vim
silent normal! 0gg/^appendix
df=xd$nf{v%p
silent normal! 0gg/^part
df=xd$nf{v%p
silent normal! 0gg/^chapter
df=xd$nf{v%p
```
Then touchoup eu, hr, lt