Support proper i18n pluralization

We should use __n for numeric messages to support unlimited, language-specific messages.

More: https://www.gnu.org/software/gettext/manual/html_node/Plural-forms.html

[kauffj]

@seanyesmunt please read/follow this thread

[marko-lorentz]

I will try to answer the questions raised by @kauffj:

• yes - using an ICU-based translation model means that you also need to add a dedicated “English translation” as the plural forms vanish from the code
  • Transifex proposes to upload the developer strings as language “en” to the system and have (developers or) translators create a translation for language “en_US” which will then be used by the production system (see explanation here)
  • the translators are able to view both (the “en” and the “en_US” strings) to derive the correct translation by switching the Transifex translation view to “show a second source language”
  • the strings that will remain in the source code should be the those with the most abstract plural like “I have %n% cars” in order to show the intention of the author (this form is called other in the ICU model)
• the content of the translation file will look different, similar to this simple example:

{
  "You have %count% files": "{count, plural, one {You have {count} file.} other {You have {count} files.}}"
}

• the translators will ideally not be in touch with this special format as Transifex should ask them to enter the one form and the other form separately if the target language requires these 2 forms (this will need a try I think - never trust the docu of a tool) (the Transifex docu also doesn’t mention how the zero form is supported and it would really be a mess if it is not, as you need “You don’t have a car.” instead of “You have 0 cars.”)
• in the source code, representation is depending on the library you use and how you wrap it; raw example for “Format JS/Intl MessageFormat” lib without wrapper:

const enNumPhotos = new IntlMessageFormat(
    `
    {count, plural, one {You have {count} file.} other {You have {count} files.}}
    `,
    'en-US'
  );
  return enNumPhotos.format({count: 1000});

I’m not a JS developer at all (I usually do Java backend stuff) but I’m sure this can be wrapped with a generic vararg function or fancy other stuff. Whether it is __n(...) or whatever might be a decision to make after reading through the docu of the lib - as depending on lib, there might be some more to discover, like e.g. nested replacements.