I do not think it is a matter of translation, it is simply pronounciation. In German, and some other European languages, a "J" is pronounced like the "Y" in English "yes" not like "J" in joy. Johann spoken, Yohan, Jakowski spoken, Yakowski in the same way, although spelt with a "W" the sound is "v". I have also seen words with the "Y" sound spelt with "ia". If Wilhelm was asked by an English speaking person what his name was that person would hear "Yakowski" and write it that way - poor old Wilhelm would have absolutely no idea that it had been done - he would still spell it Jakowski. I think he has been found.