The issue is not immediately obvious as there is one crucial difference between the Marketo connector and other connectors. Marketo takes all language combinations from Customer language combinations while other connectors obtain data about language custom combinations from the assigned Project Manager user (connectors only use custom XTM Cloud language combinations).
How to solve the issue?
Open XTM Cloud, go to the Customers tab and open the settings for a customer for which Marketo projects are created (a user with the Project Manager role is required!).
Display Language combinations, and then choose one of the following options:
System defaults with customized language combinations;
Customized language combinations only.
Set the necessary language combinations for the appropriate source language.
Open the Marketo plugin once more.
Then you will be able to change the source language.