I was reading about John Neumann, the first and to date only American man to be canonized, on Wikipedia, when some details (or rather lack of details) about the man struck me. The article mentions that he was born in Bohemia, and also gives the Czech and German versions of his name. However, at no point does it mention which version of the name he actually used himself (before emigrating to the US, anyway), which gives rise to my question:
Let's say you're a citizen of Bohemia around 1830. Maybe your mother's native tongue is Czech and your father's is German (which appears to have been the case for Neumann, according to quite possibly unreliable information on the talk page, anyway). What decides whether you primarily identify yourself by your German or Czech name? And if you primarily use, say, the German version of your name (say, Johannes Nepomuk Neumann), would you be able to "translate" it into Czech on a whim? (Would you even ever have seen a "Czech" version of your name if you were a German speaker living deep in the Sudetenland?) Would Czech-speaking Bohemians translate German names into the equivalent Czech "version" (say, Johannes to Jan) when talking about German-speaking Bohemians (or the converse)?
Bohemia was very bilingual at the time. This situation was very common. So let's assume John's dad spoke to him in German and his mother in Czech.
German: Johann -> diminutive (Bavarian) is Hans or Hansi. Czech: Jan -> diminutive is Honza (from the German)
So his dad may have called him "Hans" (but with a Bavarian accent pronounced "Hons" or "Honsi"
And his mom would have called him "Honza" ...so not very different, actually.
Your friends call you by the diminutive (like in English it would be "Johnny") ...so even as a grown up he'd just be "Honza" no matter whether German or Czech. And I mean had he stayed in the Czech Republic.
Wenceslas works the same way, and other names have comparisons, especially girl's names can be identical.
But your answer is: he probably went by Hans/Honza, but Jan in a formal context
Czechs would usually translate the name. Germans do the same. To this day the Czech and German versions of the wikipedia article you quoted only have one variant of the name (Jan for Czech and Johann for German). They are equivalent. Names are kept Czech or German if the person is 100% one or the other. But there is absolutely no problem going back and forth if they are mixed.
In fact, because his parents are mixed it seems by his middle name that his parents sought a compromise.
I say that because "Jan" is common for that reason. Same as Wenceslas, Peter, Michael, Alois etc.
Jaroslav, Miroslav, and many others don't really have a German equivalent.
So they may have went with a neutral first name, then because "Neumann" is disgustingly German, they chose "Nepomuk" (a Czech saint) because it's disgustingly Czech.
And finally the reason I think he went by Jan is because he was named after Jan Nepomuk. (though Germans call him Johann Nepomuk anyway... so let's just go with "He was Hansi/Honza")