Adding language switch (flag)

Site URL: http://martinadvocatcourageot.com

Hi everyone!

I've been looking to add a feature for a few weeks and can't seem to get my head around it.

My website is in French (the main section once you enter it).

My goal is to add a way to switch everything in english (on cover page and on the main site). 

I was thinking of a GB flag or at least a 'ENGLISH VERSION' on the upper corner.

Problem is, I contacted the support and they told me there was no way to do it using the build in tools, but maybe code injection was a solution.

So if anyone can help me with that, I'd love some advice :)

Thanks !