Here is some more info.
First step, go to the M4T site and download all the Spanish Campaigns you can find. Also check here.
In your single missions folder (along with the other countries), add a folder "ES" (Republican Spain), and a folder
"SP" ( Nationalist Spain). Go to any other country folder and copy the file "background.mat". Paste this file into both
the new "ES" and "SP" folders. Next, you need to find a "background.tga" for the "ES" and the "SP" folders.
Some of the campaigns that I mentioned at the top have tga's that you can use.
Next you should look on M4T for music for both sides, that you can put in a new "samples/music/Menu/ES" and
"samples/music/Menu/SP" folders.. The last step is the hardest. You need at find a Spanish speech pack to put into a
new "samples/Speech/ES" and "samples/Speech/SP" folders that you must create. This is a hard step because I
can't find a location to download a Spanish speech pack. If I knew how to up-load mine, I'd do it, but I have no knowledge
on how to do this. It would be nice if somebody would put a Tutorial in the Tutorial section on how to do this.
Finally, create some aitcraft folders in each country and start using the FMB to make missions for these aircraft.
Maybe somebody else will upload their Spanish Speech pack, or knows where you could download one.
PS. I use the same speech pack for both Republican and Nationalist Spain.
I wish I could explain this better, but my computer knowledge is rather limited.