bug#37327: Simplifying access to built-in tutorial

[Sebastian Urban]

> > English is still the main language for learning to use Emacs; the
> > tutorials in other languages are mainly for people who have
> > difficulties in understanding English (...)
>
> (...) more importantly, the English version (...) is maintained by
> the Emacs developers, so is always up to date; (...)  the
> translations are updated by translators if and when they become
> available, and so could potentially become outdated (...)

1.  English version is still among languages.  Additionally you could
change its name to "English (default)" or "English (official)" to
emphasize that it is recommended one.  You could even add it as
default argument - "Language (default English):".

2.  If English version is the recommended way, then why 'C-h t' loads
by default version that corresponds with user language environment,
i.e. if I type it I get Polish version?  If English version is most
up-to-date it should be loaded by default no matter what (I think
Emacs 25.2 behaved this way), and then the rest should be optional and
only available through 'help-with-tutorial-spec-language'.