JavaScript Drupal.t and Drupal.formatPlural only works for non-english languages

Problem/Motivation

The JavaScript Drupal.t() and Drupal.formatPlural() functions require the plural delimiter. They fail when the interface language is English because the plural delimiter character is added to the autogenerated JS file files/languages/<lang_code>_<hash>.js, which is only added when the interface language is not English.
Without this patch the Drupal.formatPlural() always returns "undefined".

Some background info on the plural delimiter

The database field {locale_target}.translation contains the singular and all the plural translations in one single string separated by the delimiter character. If the PO file looks like this:

msgid "every week"
msgid_plural "every @count weeks"
msgstr[0] "co tydzień"
msgstr[1] "co @count tygodnie"
msgstr[2] "co @count tygodni"

Then the {locale_source}.source field will contains a value like this:
every week$DELIMITER$every @count weeks
and the corresponding {locale_target}.translation field will contains a value like this:
co tydzień$DELIMITER$co @count tygodnie$DELIMITER$co @count tygodni
This value goes into the autogenerated file. Of course the autogenerated file is only used for the non-english languages, but the Drupal.formatPlural() does not know anything about the current language and a return value of the Drupal.t() have to be splitted by the $DELIMITER$ even if the language is English.

Proposed resolution

We have to add the plural delimiter character to the drupalSettings always, not just for the non-english languages.