When accessing Section translation on test wiki, suggestions are shown for the English -> Bengali language pair. Currently, when starting a new translation is not possible to select Bengali to search an article to translate from this language. Searching for "Bengali" to select it, leads to no results:

The expected behaviour would be for Bengali to be available to be selected.

My guess is that the default target langue is Bengali and the source selection is prevented for Bengali to avoid a language pair with the same value for both source and target. However, a better approach to prevent this is to swap source and target values when this happens (similar to T287855). That is, allow Bengali to be selected and make English (or whichever language was the source) to become the new target language. In this way, users have always all available options to select and invalid selections are avoided.