I think you should not worry too much about other people's opinion. Some people won't get it, mostly because they've probably NEVER HEARD of things like this - these kinds of allergies are not very widespread here and they don't get too much media attention. But I doubt anyone will think worse of you because of that. If someone keeps insisting on treating you with potentially deadly food tell them a horror story about "almost dying" after trying it... if it does not impress them just refuse firmly. Culturally speaking there's nothing offensive about not eating something that you can't tolerate, if you are polite about it. Even famous Russian "force-vodka-drinking" (which you can encounter among a certain kind of people) can be avoided by claiming that you can't do it because of health issues.

What can be much more inconvenient and troublesome is finding acceptable products and foods, especially if you are eating out. Russian eating establishments are pretty traditional in this regard, meaning that special menus for people with "exotic" allergies are rare. You should be extra vigilant reading labels and menus, and stressing the importance of avoiding certain ingredients if you are eating with a family.