How do I update the submit button and error message to reflect local language (i.e French)

All the above will work, of course, but they permanently change the locale of a form.

Many companies work with leads with different preferred languages (obviously!).  Ideally, such companies would not maintain multiple forms solely for localization.  (It's already hard to maintain too many forms even without localization, thus the "global form" concept!)

If you do business in multiple languages, the goal (IMO) should be to temporarily localize a form based on the preferred language of the lead. A cool solution uses the Forms 2.0 API and advanced form setup.

First, create a rich text area on the form that stores all your translations, like this.   This allows all the languages to travel with the form (you could easily maintain the translations in a separate JavaScript file, but let's look at the all-in-one method for now).

Then, whenever you render the form -- you'll see in my demo that I'm using an embedded form, and as you know anything that works in embedded mode works in LP mode as well -- you use some standard JS to check the current language and translate field labels/placeholders/validation text appropriately.  The code block is the exactly the same for every form (it could also be saved to its own file to include with <SCRIPT src=...>).

For example, here you see a single Marketo form rendered differently by passing en-US (English-US) or fr-FR (French-France) as a query param:

• http://codepen.io/figureone/pen/mJWgjE?editors=001&lang=en-US (translates the LastName's label and validation text for en-US)
• http://codepen.io/figureone/pen/mJWgjE?editors=001&lang=fr-FR (translates the FirstName's label, placeholder, and submit button text for fr-FR)

this is super simple. You just need to select localize French (under form settings) and if you don't like the way Marketo has translated French, you can customize on a field basis. Basically select field and update label and error message / instructions

Edit the form -> Form Settings -> Locale. And you can also go into the individual fields to overwrite the error message.