If your Marketo sync user is set to a language other than English you may run into an error when trying to enable a custom object sync.
Log in to Salesforce using the marketo sync user.
Under the user name, go to Setup.
Under Personal Information, click My Personal Information.
Change the Language to English.
Back in Marketo, under Admin > Salesforce > Objects click Refresh Schema.
This will pull the objects list in English. Now select the object of your choice and click Enable Sync.
Notice your custom object is now enabled and synching.
Now go back to Salesforce and use the above steps to change the sync user back to your preferred language.
Don’t forget to Refresh Schema one last time to pull the objects back in your language.