The `DELETE_X_BLOCKS` message still exists in the current version of Blockly, so I don't think this is actually an issue with the context menu.
Are there any messages in `Blockly.Msg`?
How are you importing Blockly? If you haven't loaded the messages before you inject the workspace, you might get this error. Double check your import statements against
the docs here. If you're using script tags you just need to load the appropriate message file, but if you're using imports then you also need to call `setLocale` before you inject Blockly.
Best,
Maribeth