How do you combine forms together once you have created them separately?

It is not currently possible to combine two separate forms into a single form.  It is possible to copy fields from one form to another.  You can achieve this with our app "Field Manager": http://apps.jotform.com/app/field_manager 

You can also visit http://fieldmanager.jotform.io/ 

Field manager app will give you flexibility to copy/clone one or more field from one form to another.

If this was not what you meant by combine forms, let us know a bit more detail regarding your needs and we will be happy to assist you further.

I wish to add the data from one form (in French) to the English form.  All fields are identical except for the language.

You can use the form translation feature to do so:

https://www.jotform.com/help/298-How-to-Make-your-Forms-Multilingual

Some fields and form elements may need to be manually translated.