Do you actually want to maintain the characters to ensure proper German spelling?
In that case, ensure that you load the data into KNIME using UTF-8 encoding. Using the CSV Reader as example, go to the encoding tab to change the character set.
Yes, change to the Multi Column version of the same node.
Select all columns you want to have the change applied to. Use the $$CURRENTCOLUMN$$ variable in the function. This will ensure all selected columns are updated accordingly.