TimeTap provides you with an invitation templates upon your account creation ("New Client Welcome Message") and you can add your own templates as well.
If you want to edit the default language on any template, follow this guide.
Step 1: Go to Messaging > Invitation Templates:
Step 2: Click the "View" button next to the template you'd like to edit:
Step 3: Click the "Edit" button next to the "Template" section on the template profile:
Step 4: Make the changes you'd like to make to the template and then press "Save":
Now when I go to email my clients template, if I select that template I will see those changes reflected: