Switch between english and japanese language in DLsite
< Feedback on Toggle Dlsite language
May be a problem with the Event Listener "DOMContentLoaded", try deleting/commenting the line
document.addEventListener("DOMContentLoaded", function(){
and the last line and see if it works.
My environment: Win7SP1, Waterfox 2020.07 x64, Tampermonkey
The script has not any effect to dlsite pages but added an underlined word "Toggle Site" that can't be clicked.