[3.0] Stop the forum offering to delete the language it falls back to

[albertlast]

Description

The language editor offers to delete the language the forum is running on, and
goes through with it.

template_modify_language_entries() guards the button with:

// Allow deleting entries. English can't be deleted though.
if (Utils::$context['lang_id'] != 'english') {

Language ids have not looked like that since 2.1 — Sources/Lang.php carries
'english' => 'en_US' in its map of old names to new ones, which is exactly why.
$lang_id comes from $_GET['lid'], so on any 3.0 install it is en_US,
de_DE and so on, and the comparison never matches. Languages::modifyLanguage()
spells the check the same way when it acts on the post:

if (!empty($_POST['delete_main']) && $lang_id != 'english') {

so the button is not merely cosmetic.

Deleting en_US is worse than losing a translation. Lang::load() falls back to
it for any string the chosen language is missing:

&& 'en_US' !== Config::$language
&& 'en_US' !== $lang
) {
	$attempts[] = [$dir, $name, 'en_US'];

A forum without it has no backstop.

The guard now covers the forum's own default language and en_US, decided once
where $lang_id is settled and read by both the action and the template rather
than written out twice.

Testing

On a clean install of release-3.0 (a7ac468b1), one language installed,
?action=admin;area=languages;sa=editlang;lid=en_US:

	delete button	other buttons
before	offered, with its confirmation	Save, Reset
after	absent	Save, Reset

I have not exercised the positive case — a second language still being
deletable — because installing one needs to reach simplemachines.org and this box
has no outbound access. It follows from the same expression, and en_US is the
only value the negative case can take on a single-language install.

Found while converting this template's inline confirm() to the shared
you_sure handler in #9393.

Issues References (Fixes|Related|Closes)

Related to #7933