Cell language type confusion

[kieferrm]

Testing #6821

1. create a new untiled notebook with 2 code cells; both of them have the language type "plain text"
2. install the jupyter extension
3. change the language type of the first to "python"
   -> kernel updates to Python3 (although there is no python on the machine)
   -> the second code cell remains "plain text" although the "kernel" is Python3 and therefore monoglot

[DonJayamanne]

Should not be an issue anymore, moving to verification