Add ability to specify a different default editor font per locale

[mkaz]

Description

Allow translators to specifying a different default font based on the language/locale. The translator can specify a different Google font to load, and what the CSS font-family should be for different locales.

Fixes #9891

Types of changes

Adds two new translation fields to allow language translation to specify a default font.

The first field is Google Font Name and Variants which can be set to off to disable loading an external Google font. The default value is: `Noto Serif:400,400i,700,700i'

The second field is CSS Font Family for Editor Font which is the css font-family name specified in the Google font loaded. The default value is: Noto Serif

This includes the variants because certain fonts may not support italics, for example Noto Sans JP.

The second field could also be used to specify a default system font.

How to Test

Confirm the Editor default text is using the Noto Serif font.

1. Generate a gutenberg.pot file using makepot.php from core See i18n

2. Copy gutenberg.pot to gutenberg-es_MX.po

3. Edit gutenberg-es_MX.po and change the fields, here's the example I used

#: lib/client-assets.php:768
msgctxt "Google Font Name and Variants"
msgid "Noto Serif:400,400i,700,700i"
msgstr "Pacifico:400,700"

#: lib/client-assets.php:1567
msgctxt "CSS Font Family for Editor Font"
msgid "Noto Serif"
msgstr "Pacifico"

4. Generate the mo file using msgfmt -o gutenberg-es_MX.mo gutenberg-es_MX.po
   If you use Homebrew on OSX, you may need to run brew link gettext --force to add the msgfmt binary.

5. Go to Settings > General and change your site Language to Espanol de Mexico
   This may require first adding define( 'WPLANG', 'es_MX' ); to your wp-config.php.

6. Finally confirm the default editor font is changed to Pacifico, which is a distinct script font.

You can obviously test using a different locale and font to match what works best for your language.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[georgeh]

Tested and confirmed that I was able to change the editor font using localization tools.

If this becomes a more common thing, we may want to look at making localized CSS part of the build process, but for now this should solve the current issues with minimal overhead.

[mkaz]

Core ticket: https://core.trac.wordpress.org/ticket/45382#ticket