Make it clear why languages are not available for MT

[MizukiTemma]

Motivation

In the bottom of " Select provider" box in "Manage machine translations" section, languages are listed that are not available for machine translation.

However the reasons are not mentioned. There are in most cases two possible reasons: 1) the languages are not supported by none of the available MT providers, and 2) "Machine translatable" is not checked in the language node setting.

This is probably confusing for users. Make it more self-explaining to reduce potential enquiries to the service team.

Proposed Solution

Add a short explanation about the possible reasons, or list those languages grouped by the reason.

Alternatives

Leave it as it is.

User Story

Additional Context

Design Requirements