Thanks!
The UI was made with QT Designer, which is a very easy to use, graphical form/widget builder - it took me maybe 30 minutes to create the form above as far as layout goes. There is a Python plug-in called PyQT that then takes the QT UI file and translates it to a Python script, so all the actual functionality of the buttons/comboboxes etc. can be done in the Python language.
The functionality part of the above form is still a WIP - it forced me to re-do how I was storing the settings for the generation stuff, using Python's pickle function which I didn't bother learning before because I didn't see how it would be useful... until it turned out to be exactly what I needed, as seems to often be the case.
Re: the name files, I have 80 nations done so far, some I put together manually for FBB over the years, but most of the others were done by text-scraping wiki team pages for domestic soccer leagues, as a surprising number of English-language wiki pages have a detailed rosters. There's still a manual step of going back and deleting out foreign players from each league, but it's still quicker than the old "copy+paste+edit" method I used to use.
The current plan is to have 100 different nations available, but that will change when I start to look at World Cup qualifications way, way down the track, making sure I have a balanced amount of teams per the 5 FIBA regions.