Create a test plan, add parameters to this test plan, change language, all parameters are removed...
Need a bit more detail on this. Which parameters? Does it happen with all test plans?
Sorry, parameters is not the right term. The problem occur with 'User Defined Variables' in the root TestPlan item. This list is emptied when modifying the language.
OK, I see what you mean now. Perhaps the run-time language change option should be removed for the time being - it does not work with all test elements anyway.
This seems to have been fixed by the changes to the various clear() methods.
This issue has been migrated to GitHub: https://github.com/apache/jmeter/issues/1924