Source language different from project source language

I know this is a very basic question but it's driving me nuts!

When I browse for files, to create a new project, I select the files I want (NL-BE or NL-NL to EN-UK) and I always get the error message "The source language of the file is different from the project source language". The project window then opens and I have to go back in and select the files again, once I've changed the source language in that window. 

I can always create the project in the end but can someone please explain to me how I can just pick up EITHER Dutch NL or Dutch BE files and create projects without the faff.

Thanks :)