Answer
No — Custom Event/Event Action error and warning text isn't picked up by the standard language-text translation facility the way built-in application messages are. A practical workaround for a small number of target languages: check the current session's language via FND_SESSION_API.Get_Language inside the event action's PL/SQL, and conditionally call the appropriate language-specific error/warning text based on that value — effectively hand-rolling per-language message branches rather than relying on automatic translation lookup. This approach is only practical when the number of languages that need covering stays small, since each additional language means another conditional branch to maintain.