What can I do to make sure I don't get sick eating fish in Japan (as a 11-year vegan)?

I'm planning to visit Japan next month and am a bit concerned about eating. I've been a vegan for 11 years and I'm going to try my best to avoid meat, fish, eggs and dairy while I'm there, but because of the language barrier I'm concerned that I'll accidentally be given something with fish or fish sauce. I'm not so much concerned about the ethics of the Japanese food system (especially since as an outsider I don't think it's my place to judge), but I AM concerned that my body won't know how to process these foods it hasn't seen for more than a decade and I'll get sick. I do get sick in the US when I'm given eggs or dairy by accident (though no one has accidentally given me meat in about 10 years).

Is there something I can take (I'm thinking along the lines of how lactose intolerants can take lactid AC to avoid a bad dairy reaction) to make it so I don't get sick from eating fish?