That's interesting Esther because I'm in Ireland, the main language of my site is English and yet I still default to myheritage.es. I don't mind this but I tried to send the link to my brother in Cornwall and he gets it in Spanish too! So there does appear to be a way of disabling the IP following.
What you can do is bookmark your site with ?lang=EN in its address.
For example, bookmark this:
What this will do, when visited, is always go to your site and FORCE the language as English. It will not switch to French, which it might otherwise try to do because of your location.
Another solution is avoid logging off every time. When you stay logged on, the risks are very minor, and cookies remain and remember your language preferences. This way visiting myheritage.com will keep you in English. But you have to log off if someone else is using MyHeritage.com on a different account on the same computer.