There is a long tradition of writers who have chosen to write in a language other than their native language at some point in their lives (here I am excluding writing in the scientific language of the time). Examples range from the medieval philosopher Avicenna, who wrote in his native Persian as well as Arabic, to modern writers such as Nabokov and Kundera. Another example is the Italian writer Antonio Tabucchi, who wrote a book in Portuguese after living in Portugal for many years. The book is Requiem, Uma Alucinação (1991; second edition, 2012; Publicações Dom Quixote).
This book and its sequel, Per Isabel, belong to a genre of Portuguese books featuring the ghost of Fernando Pessoa. I’m not sure if this genre really exists or if I’m inventing it 😀.
In any case, long story short, Tadeuz Waclaw is a character in Requiem, and I chose it as my GitHub username simply because I loved that book. It was a rather random choice.