There are some historic examples for people who used (or had used for them in the literature) different first names depending on language and culture at their time. Example: the son of Spytihněv II of Bohemia (no profile yet) who is variously referred to as Svatobor, Fridrich, Friedrich, and Gunther. They are all demonstrably the same person, the only Slavic patriarch of Aquileia:
- Svatobor Friedrich, Charles Cawley, Medieval Lands
- "... Guntherum patriarcham ...", Genealogica Wettinensis
- Svatobor-Bedřich, J. Čechura, J. Mikulec, F. Stellner, Lexikon českých panovnických dynastií, Prague 1996
- Swatobor (1084-1086): Il Patriarcato d'Aquileia - Patriarchi "Ghibellini", infinito.it
- Friedrich II. von Mähren, Swatobor Friedrich von Böhmen, Federico di Moravia, Svatobor (Fridrich) in German, Italian, and Czech secondary literature
As a member of the Czech Přemyslovci dynasty his Proper First Name should be Svatobor but what about the other "first" names he is known under? Leave them to the biography?
Addendum: More recent applicability - people from bilingual areas who have used their name in both languages at the same time depending on the circumstances (my grandfather used to carry two birth certificates, one in German with his name as Johann, one in Czech with his name as Jan).