Defining source language in machinery backend

[nijel]

Describe the problem

Some machinery backends support only certain language pairs and not all languages. Cyrtranslit could be a typical example.

These backends fail to perform the translation when the target language is supported, but the source is not. In such case, it could automatically change the source language to a supported one if it is available.

Describe the solution you would like

• The implementation depends on Using other than source language in machinery backends #13768.
• It can extend the language fallback mechanism to choose supported one instead of automatically choosing source.
• It could probably take precedence over user supplied language as well when it is not supported.
• The scope of this should be limited to simple engines such as Cyrtranslit - when there it is clear that the engine can't select anything else as a source language to work.

Describe alternatives you have considered

No response

Screenshots

No response

Additional context

No response

[github-actions]

This issue has been added to the backlog. It is not scheduled on the Weblate roadmap, but it eventually might be implemented.

In case you need this feature soon, please consider helping or push it by funding the development.