you can completely customize that login page to whatever you need.
There is no simple pull-down menu to choose a language from.
But there is more powerful solution .. "Replacement Messages" in System menu.
By default in Simple view mode there is just a few, including SSL VPN Login Page. However top-right switch can be used to Extended view and there will be a lot more to see and play with.
This is a place where you can see, and modify messages FortiOS will use to interact with admins and users in GUI.
And you can modify those. Then wording and language is up to you.
As you will see you can change HTML code for selected page. Also view result in real time.
And if something goes haywire you can always reset to factory content and start over.
Tom xSilver, planet Earth, over and out!