Follow the next steps in order to change the standard title for Contact Form Submit button:
- Go to Contact Form plugin settings page.
- Select "Additional settings" tab.
- If necessary, add a language in the block "Language settings for the field names in the form".
- Mark the checkbox (if it's not marked yet) in the block "Change the labels of the contact form fields and error messages", choose "Default" tab, click the twistie to expand the list of the fields and fill in "Submit" field.
- Save changes.