Pablo Casals is also correct, since that is the way he was know to most people, and probably was his mother call him.
He's mother was from Puerto Rico, where he live part of his life, died and was buried. Early 1900's pretty sure people couldn't pronounce his birth name of Pau, so people call him Pablo. He married a musician from Puerto Rico, he took permanent residence in Puerto Rico.
To this day, in Puerto Rico they have a yearly week long Pablo Casals Music Festival, which himself started back in 1955. In 1959 he created the Music Conservatory in Puerto Rico. To all those people with whom he was involved and whose live he touch, he was know and call Pablo.
In any case, what he was named or called is not what is important. What is important was his talent and his legacy.