When Hitler invaded Yugoslavia, the Royal Yugoslav Air Force were using Bf 109Es against the Luftwaffe.
Heres a site that lists the Yugoslav aces...
Air Aces Homepage
Neutral Switzerland also had German machines that were used to protect their borders. Whenever hostile aircraft, such as German fighters or bombers crossed into their airspace, a few battles raged.
A quik ref from wiki......
"On 10 May 1940, air combat between Switzerland and Germany was initiated. Several Swiss Bf 109s engaged a German Dornier Do 17 near the border at Bütschwil; in the ensuing exchange of fire, the Dornier was hit and eventually forced to land near Altenrhein.
On 1 June the Flugwaffe dispatched 12 Bf 109 E-1s to engage 36 unescorted German Heinkel He 111s of Kampfgeschwader 53 that were crossing Swiss airspace to attack the Lyon - Marseilles railway system. The Swiss Air force sustained its first casualty in the engagement when Sub Lieutenant Rudolf Rickenbacher was killed when the fuel tank of his Bf 109 exploded after being hit by the Heinkel's return fire. However the Swiss Emils shot down six He 111s.
On 8 June, a C-35 observation aircraft, an antiquated biplane, was attacked over the Jura Mountains by two German Bf 110s; the pilot and observer were killed. Later on the same day, Swiss Captain Lindecker led about 15 Swiss Emils to intercept a formation of German He 111s escorted by II./Zerstörergeschwader 1's Bf 110s. The engagement resulted in five Bf 110s being shot down (including the Staffelkapitän Gerhard Kadow) for the loss of one Swiss Bf 109.