Hice un máster en la Universidad de J., pero no puedo especificarlo como tal en mi currÃculum porque nunca estuve inscrito oficialmente. Como máximo, puedo indicar "alumno libre en el máster de la Universidad de J.".
Remarks about the translation
alumno libre = oyente; el alumno que acude a las clases sin estar matriculado y, por lo tanto, no supera ningún examen y no recibe ningún tÃtulo.
I have attended a Master’s at the University of J., but I can’t put this in my CV as such, because I never officially enrolled. At most, I could indicate “Occasional student at the University of J.’s Master’s programmeâ€.
Remarks about the translation
University of J. -> or: J. University, depending on which university this is.
‘ I have done a master’s ‘ implies the course was completed successfully. Since that is not the case here, it would be better to replace the first sentence with ‘ I have followed a master’s course’ or something similar. I did not put this in the translation because in my opinion, the same applies to the Spanish text.