I was split between で or は, forgot all about が. Funnily enough I think omitting the particle altogether would have made more sense. Edit: After further reading

I was split between で or は, forgot all about が. Funnily enough I think omitting the particle altogether would have made more sense. Edit: After further reading は seems to work fine. In this case I think both work but が places greater emphasis on the german language being the thing not understood. で however was totally incorrect :P. But due to the tacit nature of informal Japanese I think the context already informed the reader who doesn't understand what.