https://stackoverflow.com/questions/64860/best-way-to-convert-text-files-between-character-sets

Gnu recode (manual) suggested by Cheekysoft will convert one or several files in-place. Example:
   recode UTF8..ISO-8859-15 in.txt

   recode ISO-8859-15..UTF8 Ungarisch-Deutsch-ISO-8859-1-160413-c.csv