There are many unique and interesting units that served in WW2. I find the best way to incorporate them into this game is to make them special squads, Premium or Events.
While Spain was sympathetic and ideologically aligned to the Axis the country did not enter the war because it was still reeling after its civil war and was in no state for mass military conflict. However Spain did send thousands of volunteers who wished to join Germany in its great invasion of the Soviet Union, the Blue Division
I think a Premium/ Event squad’s weapon should be thematic, such as the Australian squad having the Australian Sten. There are some good candidates for this squad’s weapons. They would be an Assault squad
Labora Fontbernat M-1938. It was a submachine gun fabricated and used by the Republican forces of the Spanish Civil War. I would see this weapon’s inclusion similar to the Chinese weapons that are Gold Orders for Japan like the FN Model 35. We can reason that these are captured trophies from the earlier civil war. It’s magazine is 36 and RoF of 750
Star Si35. It was an indigenous design made under fascist Spain to finally develop its own arsenal instead of foreign purchases. It had either a 30 or 40 round magazine. The weapon came in 3 different variants. The Ru-35 had a RoF of 300, the TN-35 had a RoF of 700, and the Si-35 could toggle between both RoF. So I would say most likely we would have the Si-35 because it has a unique functionality and basically a combination of the Ru and TN, best of both worlds
There is also a potential for a second Spanish Volunteer Squad. In the fall of 1943 Spain had ordered its volunteer forces to return, but some thousands did remain to continue fighting with Germany. Some were absorbed by various German units and even the SS. These various Spanish volunteer holdouts were collectively known as the “Blue Legion”
A potential weapon could be the Star Z-45, a Spanish derivative of the MP 40. The first prototypes of this gun were made and tested in 1944. It had a 30 round magazine and RoF of 450
I will also say these weapons could also be added as Gold Orders since they are one of many unique miscellaneous guns of the WW2 era.
A Spanish Squad(s) would have a unique appearance as well