General Testing Instructions

Create a multilanguage site, associate some articles (here in French/English), leave some articles non associated.

Test

Log with a Super User, edit an article for example in fr-FR and leave it in edit state (Here : "Newarticlefrench"). The article is now checked out.

Create a Manager with access to Multilingual associations and log with another browser.

Choose in the Multilingual associations list an Article tagged to English.
It is already associated here to "Article (fr-fr)"

Then click to open the side by side. Choose French as Target language. Then use the "Change Target" button.
The modal displays with all articles:
Note that "Newarticlefrench" is NOT marked as checked-out and can be chosen by the Manager.

If selected, we get:

What this patch does

It lets choose in the modal the already associated article OR another one which is not checked out.

Patch, create another article and save it. Here "another article french", then do as above.
You will now get

"Article (fr-fr)" (the original associated article) can still be chosen, as well as the new article "another article french", but not anymore "Newarticlefrench".

@rdeutz and all.