Assuming you mean the pop-up messages from Magix Connect you can change the language settings as shown below, you can also turn off those messages you do not want to receive regarding Magix products on the Info channels tab.
If you mean in program messages, then we need more information about which products, version number(s) and variant ie Basic, Plus or Premium, you have installed - see here for more information, also what language settings are set in Windows?
. . . . upgraded 2014 premium with 2020 premium . . . . and the serial number starting with P3 will not work . . . .
Please note this is a user to user forum not Magix Support and to achieve a solution for you it would help us to help you by giving more detail in your responses, otherwise we are going to be going backward and forwards with more questions and requests for clarification as seen below.
Do you mean:
the serial number you received by email when your purchased the upgrade to 2020?
if so then
are the messages are in German when trying to register/activate 2020?
if so please post a screen shot of the message
did MEP 2020 replace 2014
We still need the other information requested to locate where the language issue is coming from.
Can you post a screen shot of the message
The Magix Connect icon will be in the system tray bottom right of the Windows screen as shown below