Do people who were from Slovakia or surrounding countries who were or are named Wilhelm or William or Vilem or Viliam often go by the nickname of Walenty or Valentyn or Valentin or Valentine?
One of my charts, for my uncle's ancestors, shows consecutive men whose original name was a variant spelling of William, but they sometimes were known as a variant spelling of Valentine in the United States, even in some church records, where I wouldn't think nicknames would ever be used.
Maybe the explanation is not a nickname, but something else. The names don't seem to go together, to me. Maybe it's something like how some men named John go by the name Jack, most notably, President Kennedy.