The MOCO Plugin creates the necessary customfields:
and gives them the prefix MOCO. If you wan't to change or translate those names, you can do this in the Jira administration.
Go to Administration → Issues → Custom fields. To simply change the name of a custom field, select the "edit" option and type in the new name of the field.
If you want to add translations for different languanges select the "translate" option.
Do not delete the custom fields or create them on your own. The IDs will be stored inside the plugin, to ensure there is only one field of each type.
If you messed things up, you have to reinstall the plugin.