Having 2 "edit survey setting" makes it more complicated than it is, because what you want is to be able to make the welcome message page editable immediately, depending on your selection in the available language choice (by using the drop-down menu) (just like how you change the "surveys" or "question groups").
That is, if you put everything in one page (with both editable welcome message and the ability to add/remove languages in the same page), then add a drop-down menu for the currently available language.
Then the user will be able to pull-down the menu and select their language of choice to change the welcome message.
If they don't want to change the welcome message, they can just add/remove the language right there. Once they add/remove the language, the page should refresh to update the new item-list in the drop-down menu to reflect the new addition/deletion of available language.
Let's say they want to change the welcome message of their Spanish survey, all they need is to pull-down and select Spanish in the menu. Then it will refresh to the Spanish welcome message editable page.
If they want to add Dutch to it, they can do that in that same page to add it. Then it refreshes to include Dutch in the drop-down menu. Then they can change the welcome message immediately (without having to page through different tab or icon to do that). It will be all-in-one page. (If I understood how the language setting correctly.)