Localization keys for notification templates should include variables

The localization keys for notification templates should include variables.

From the comment here on XWIKI-20363:

Additionally, unlike European languages, in Japanese (and Korean as well) there is a concept of particles which are supposed to be added between Subject and Verb.

See: https://www.fluentin3months.com/japanese-particles/

This means that splitting a sentence into a subject and verb(s) with two different keys does not really work.

For example, the key "mentions.event.mention.description", which has "mentioned you on page" as the original strings, seems to expect a subject to be added as prefix such as "John mentioned you on page". This should be avoided.

Instead, a variable should be used with a complete sentence. This case it should be "{0} mentioned you on page", so that translators add a particle between Subject and Verb.

Please check this comment too.