This is a very tricky problem. Let me explain:
Text is printed on the certificate with a funcion cert_printtext(), wich is defined in mod/certificate/lib.php. This function uses a function Cell(), wich is defined in tcpdf.php, the part that generates the actual pdf-document.
Cell() seems to support only single lines.
tcpdf.php knows other functions that can handle linebreaks, such as MultiCell() and WriteHTMLCell().
MultiCell() can write longer text and adds an automatic linebreak if the right margin is reached. The sequence \n forces a linebreak.
WriteHTMLCell() allows HTML code to be rendered fairly correct.
I have added new functions cert_printmultitext() and cert_printhtmltext() into my mod/certificate/lib.php, wich are basically copies of cert_printtext() but invoking MultiCell() and WriteHTMLCell() instead of plain Cell().
With these functions I can define types of certificates with more text, e.g. using the content of the intro-field as a summary of the lesson
Another idea is using the WriteHTML()-function that has been added to Version 1.9. This is used for the customtext field for the teacher's name. But I have no further experience with this function.
Of course it is always possible to position the first and the last name in two different lines (such as "Señor Pedro" above "Nombre con Muchas Palabras Bonitas"). Reducing the font size is also an option. Or are spanish names even that long?