Plugin Directory: Locale banner is not always displayed when it should

[SergeyBiryukov]

Viewing a plugin page in the English directory:

• If a non-English locale is detected from browser headers and the plugin is translated into that locale, a notice is displayed: This plugin is also available in [language name]. So far so good.
• If a non-English locale is detected from headers, but the plugin is not yet translated into that locale, there is no notice. To get more translators to contribute, there should be the same notice we get in a localized directory: This plugin is not available in [language name] yet. <a href="%s">Help translate it!</a>.

Related: #2921

[SergeyBiryukov]

In 6030:

Plugin Directory: Display locale banner if a non-English locale is detected from headers, but no translations exist yet.

Fixes #2922.