Not sure how you would identify "the first Japanese person" to cross. Very hard to sort them out from other Asians these days, esp as they are now in a minority. Even going to a "Japanese" restaurant or store is problematical because there is not likely to be any J people there. Even most "Japanese" WLs aren't these days. How personal is this letter? Is it something you would want strangers to read? Professional translators are probably expensive; try googling for name/contact details in your local area.