Arabic جلال (jalāl) is a well-established part of the triconsonantal root system, built from the root JLL "greatness, magnitude, height".
"Excellent" meanwhile has cognates all across the Indo-European world (in Italic, Germanic, Hellenic, Balto-Slavic, and possibly others), which all trace back very nicely to a reconstructed root *k-lH "to be tall".
So if there was a borrowing or relationship, it happened long, long ago, back before Arabic and English existed, in the distant era of Proto-Semitic and Proto-Indo-European. (Otherwise we wouldn't see it so well-integrated into the descendant languages.)
Is it possible that Proto-Semitic and Proto-Indo-European influenced each other, or one borrowed the word from another, or they both got it from the same source, or they both descend from a common ancestor? Sure! Could this word be evidence for a relationship? Absolutely! Especially since, if I remember right, Arabic j descends from Proto-Semitic *g, a velar.
But, and this is an important "but", it takes a whole lot more than one word to establish a relationship between two languages. Languages have a lot of words in them, so the law of large numbers says you're practically guaranteed to find some similar-looking words between any two languages if you look hard enough. And that applies to Proto-Indo-European and Proto-Semitic even more than it applies to modern languages, due to the inherent uncertainties in the reconstructions. It's not hard to come up with a dozen coincidental similarities between Swahili and Japanese, or Basque and Nahuatl, or any two languages you pick.
So this could be evidence for "Indo-Semitic", or it could be a complete coincidence. And until you have a whole lot more evidence, hundreds of words at least with clear phonological (and ideally morphological) correspondences between PIE and PS, the null hypothesis is that the two are unrelated.