Standardize and localize the <title> on login pages

[jonoaldersonwp]

The <title> tag content on all pages/states on login.wordpress.org should append | WordPress.org.

When a localized variant other than the default/global flavour is requested (e.g., ​https://login.wordpress.org/?locale=ja or ​https://login.wordpress.org/?redirect_to=https%3A%2F%2Flogin.wordpress.org%2F&locale=af), the <title> value should be adapted to append the localization name to the end of the string.

E.g., on ​https://login.wordpress.org/?locale=fr, the title is currently Connexion à WordPress.org, but should be Connexion à WordPress.org | WordPress.org Français.

[dd32]

In 10829:

Locale Detection: Some language codes do not follow the xx_XX format, and xx_xxxxx is valid, likewise some locales have variants, allow for those too.

This affected locales such as art_xpirate which would revert to art_emoji or ca_valencia which reverted to ca.

This also affected de_CH_informal which would revert to de_DE, or de_CH_404 which reverted to de_DE instead of de_CH.

See #5662.

[dd32]

In 10830:

Login: Append the Locale sitename to the login title.

Fixes #5662.

[jonoaldersonwp]

Great job! :D