Automatically choose close languages as source for machine translation

### Describe the problem

* Machine translation usually works better for close languages as they have similar grammar.
* Machine translation also might not support current language pair, but the translation is possible from other language.

The backend should choose the best language to translate from automatically. This is an extension of #13769 to make it more versatile.

### Describe the solution you would like

* The implementation depends on https://github.com/WeblateOrg/weblate/issues/13768.
* The source language selection should select the best source language based on the following info:
* List of supported languages by the backend.
* List of language groups that are close (perhaps we can get this out of CLDR, or build the groups manually).
* Availability of existing translations in the current component.

### Describe alternatives you have considered

https://github.com/WeblateOrg/weblate/issues/6232 could work when doing one translation, but it is more convenient to let Weblate choose the best approach to perform the automatic translation.

### Screenshots

_No response_

### Additional context

_No response_