For the time being the country of issue doesn't matter. Remember that EASA is not an EU organization. It is simply the European body that is responsible for aviation and space.
I have done all my training in the UK and it didn't seem to bother them.
You need to have a good command of the German language though, as that is the main language for testing at the DLR.
Apart from that, they are quite happy to invite you if you meet the criteria. The biggest stumbling block seems to be the DLR test. If the online numbers are to be believed about 90% fail that test. I swapped numbers with a few other guys I met at the DLR and none of them passed.
Quite frankly, I am still mystified by how I passed. When I left Hamburg I was sure I hadn't done very well.
Personally, I have an EU passport so even after 2019 I should be alright to move wherever I want. If you hold a UK passport it could be that after the transition period you will require a visa to work in the EU.
But no one knows yet how that is all going to play out.
We will see.