A few questions about WPML

1. Site language is a WP setting in WP Backend – Settings – Site language

2. Listings are added in the language of the add-listing form. Once you translate all templates you will have an add-listing form in JP and EN. You choose which one to use to add the listing.

https://wpgeodirectory.com/docs-v2/addons/multilingual/

Select ‘duplicate page’ -> into another language, yes. For additional questions about WPML you should refer to the WPML documentation.

What our addon does is make GD work as expected, like native posts, for WPML. It doesn’t add any new features outside WPML.

Each GD Page template should be translated into another language.

So after translating the site into Japanese I can then set Japanese to be the main language in the WordPress backend and it will use my Japanese translations as the default language setting is that correct?

Yes, but refer to WPML docs for how to do that.

Doing this won’t break anything link wise etc on the site?

No

This will then make English the second language option for the site right?

Yes

With regards to point 2, if you are viewing the site in English you will get the English add listing form so this will create an English language listing page obviously. If you change the site language to the second option, Japanese the listing form will appear in Japanese. So this then creates a Japanese listing page that I can translate/duplicate in to English right?

Yes