What can I expect from a two to three week "refresher course" before going to Germany?

I had to go through the exact same thing before re-integrating into the active Army. Its just a refresher course for your basic training, i.e. shooting, physical fitness, hand-to-hand combat, marching in formation again, etc. Are they now letting prior service back in? I was hoping to get back in the service as well.

Weapons quals learning the language and uniform appearance as well as some r and r

I personally have never heard of anybody going to a refresher course. I would expect it would be some of the basic fundamentals of policing (I.e hand cuffing techniques, interviewing, report writing, Domestic violent scenarios. This can be a good thing, so being National Guard, you might not do this as an everyday job. I got you message, it looks like the 18 MP BDE falls underneath the unit you mentioned. Looks like it is Germany wide, so until you get pin point orders, you really won't know where you are going. With that, try to call your Branch Manager at HRC and see what you can find out. That is once you fully reenlist.