How to set front page so new users lands on a specific language (change default language not working)?

How do I change the default front page to land on a specific language? Do I need to change the default site language or can I just change the default front page?

I've built a multilingual site in English, French, German and Spanish. I have set English as the default language and built the site in this. However, my client has now told me that new users must land on the Spanish home page first.

I have changed the default language from English to Spanish (I realise there are issues with this, e.g. menu items needing re-translating, and it is a complete NO as far as i18n sites go. However I don't think I have much choice do I?). The front page and site is still English by default.

So, I tried

a) manually changing the front page to default to /es (in admin/settings/site-information), but it won't allow it.

b) adding an /en extension to the English language and removing the /es one from Spanish, but now my default front page says "mysite.com/en" and I can only add a page or node after the /en extension.

So, my basic problem is this: the site was built with English as the default language. I now need to make sure that any new user who hits mysite.com is presented with the Spanish site first. How can I do this?

Thanks!