(setq ispell-program-name "hunspell")
(setq ispell-local-dictionary "en_US")
(setq ispell-local-dictionary-alist '(("en_US" "[[:alpha:]]" "[^[:alpha:]]" "[']" nil ("-d" "en_US,ru_RU,de_DE") nil utf-8)))

M-x flyspell-mode

ISO/IEC 14882 Draft №4606 §23.2.4 Associative containers

9 The insert and emplace members shall not affect the validity of iterators and references to the container, and the erase members shall invalidate only iterators and references to the erased elements.