Remember that some non-English names could be translated. The Swedish Anders became Andrew, as did the Latin Andreas. For non-standard names or those that had no real translation, the ancestor might never have Anglicized or might have simply taken an English name that was "close." Focke may have become Frank or Trientje might have become Tena. Trientje actually has a root similar to Katherine, but most of my Trientjes who Anglicized their name opted for Tena instead.
Remember that your ancestor was not a linguist--so don't over analyze that translation of his name.
And problems with Anglicization is part of the thing that was the issue with the family I'm working on for issue 41 for Casefile Clues.