Then, you need to test it on the physical server console (monitor, keyboard, and mouse) directly connected to the Server PC running Server 2012 software. Does it do it there, Yes or No?
Are you using Server 2012 RDP? or another VM app such as VMware or Hyper-V on the Client side? It's really hard to troubleshoot a remote client VM until you know whether or not the problem is occurring on the server console or not!!.
If the problem occurs on the server console, you have a problem with the Server 2012 itself, Windows corruption, or a failing piece of hardware on the server PC such as a RAM stick(s) or a Hard Drive or RAID array. If faulty hardware is found and replaced, retesting for the problem on the physical server console should show the problem resolved.
Do you have any other client PCs besides the lady you mention in your original Post that work with the RDP virtual machine in both languages correctly (German and English?)?? Perhaps her PCs are not working correctly, or have virus infection (you check for that or did you take her word they are virus free?). If you can prove that the server console works correctly, but not any of the RDP client PCs including the lady's PCs you are having the problem with, then you can begin looking at the RDP app as being the problem if NONE
of them work correctly.
You then might have to use a different app for your remote clients to access your Server 2012 domain login, such as mentioned VMware, Hyper-V, etc. You might also have a look at the new W10 Quick Assist which came out in August 2016 with the new W10 v1607 release. If the Quick Assist works ok, that might also prove you have an app-related fault or bad setting on your lady's client PCs or perhaps that RDP app is faulty through all the Client PCs you have connecting to your Server 2012 domain altogether?? Sounds to me like you need to do some data center testing where the Server lives in order to resolve this issue. If the server console fails to work, you have a problem in your Server 2012 computer or install; if it works ok, your problem is most likely in your Client PCs coming in through RDP. That means the Client PCs (some or just this lady's PCs) are all bad, misconfigured, or you have a network traffic issue.
Just thought of this also, do your Client PCs connect to the Server 2012 domain via a VPN, Extranet web-portal or Intranet web-portal? Is this done through a web-hosting site via IIS that is installed on the Server 2012 itself? If so, you'll probably need to remove that web-hosting IIS setup from the Server 2012 and move it to another dedicated Server for your Extranet or Intranet hosting. Traditionally, IIS doesn't like sharing it's resources with other apps on the Server PC--we found this out the hard way many years ago. This is especially true if you happen to be running an ERP app on that Server 2012 such as CS/3, Oracle, SAP, etc.
We don't really have a complete picture of your back-end that lives in your data center or wherever that Server 2012 is, so we don't even know if it's the top Global Domain Primary or not--and without that kind of information, we can only guess how your network infrastructure is setup. Some of these things have helped me solve these kinds of Client PC problems before, but starting with your Server console is a great place to start.
Let us know how you get on. <<<BBJ>>>