We've had a couple of people here use mexiletine occasionally before sports events, but that's something that would need to be monitored and approved by your doctor. Not everyone with a sodium channel mutation does well with mexiletine and I don't think it's available in Germany.
A better option for you to discuss with your doctor might be acetazolamide. I live at a high altitude and it is often prescribed for people who come out here to go skiing to help prevent high altitude sickness, so I know it is considered safe along with physical exertion. The only complaint I hear for short-term use is that it makes carbonated drinks taste funny (metallic taste). With long-term use it can cause some tinnitus, but it usually stops as soon as you quit taking it.
If you want to try an alternative to drugs the amino acid L-Taurine has been shown to help myotonia. I don't remember if it was both chloride and sodium channel myotonias. I'll try to find out. The dose would be 1-2 grams. I find that Acetyl-L-Carnitine really helps me with endurance and reduces stiffness.