Is Delong an Italian name?

No, Delong and DeLong aren't Italian names. The article 'de' means 'of, from' in French. The equivalent in Italian is 'di'. But it may have started out as an Italian name that was translated into French by Italians who decided to settle in France or French speaking areas of Italy.

For example, the Dutch last name 'Vanderbie' originally comes from Italy. The family was 'Abella' in the Genoa/Pisa area of north Italy. The name became 'Abeille' with a move into France. And then it became 'Vanderbie' with the branch that relocated to the Netherlands.
No, Delong and DeLong aren't Italian names. The article 'de' means 'of, from' in French. The equivalent in Italian is 'di'. But it may have started out as an Italian name that was translated into French by Italians who decided to settle in France or French speaking areas of Italy.

For example, the Dutch last name 'Vanderbie' originally comes from Italy. The family was 'Abella' in the Genoa/Pisa area of north Italy. The name became 'Abeille' with a move into France. And then it became 'Vanderbie' with the branch that relocated to the Netherlands.