When choosing the parameters to merge translations from another project or version, if the user doesn't change anything on the dialog, the parameters passed to the backend specify the same source and target version.
The backend only receives the correct source version when something is changed in the dialog, and in the case of a project only having two available source versions, there is nothing to change. The current workaround is to create another version so that the selection in the dropdown can change, thus triggering the right parameter to be passed to the backend.
We should change the way the parameters are being passed to the backend. Currently they are bound whenever the dropdowns are changed, which causes this problem. We should be submitting (using ajax as it is now) the complete form when the merge button is pressed. This way the parameters are passed to the backend at button press time.