Translating 不得了 as "extremely" seems pretty correct to me. It's a general intensifier in both Mandarin and Cantonese. It's a fixed expression and cannot be reduced to its component parts. In Cantonese it can be used with 到 in front; in Mandarin it's usually used with another 得, and I think it can be used like this in Cantonese as well.
"呢班人真係勁得不得了" - "These people really are extremely skilled"
Also worth noting: this 了 is not the "completed action particle" that corresponds with Cantonese 咗. That particle is read "le5" in Mandarin, whereas 不得了 is read as "bu4 de2 liao3". It means something like "to accomplish" or "to bring to a satisfying completion". "我做不了" would mean "I'm unable to do it" (opposite of 做得了). The Cantonese counterpart to this would be 到 (in second tone): "我做唔到". A literal translation of 不得了 into Cantonese would thus be 唔得到, but since it's a fixed expression it's not translated.
On its own, 不得了 means "great" or "impressive" (similar to 犀利). 嘩, 你真係不得了喎! I don't think it can be coupled with other intensifiers, so you don't say "佢好不得了".
Finally, 不得了 can also mean "terrible", at least in Mandarin/written Chinese: "如果那樣做, 那可不得了". I'm not sure if I've heard it used like this in spoken Cantonese, though.
EDITed for typo. wrote 唔, meant 咗.
"呢班人真係勁得不得了" - "These people really are extremely skilled"
Also worth noting: this 了 is not the "completed action particle" that corresponds with Cantonese 咗. That particle is read "le5" in Mandarin, whereas 不得了 is read as "bu4 de2 liao3". It means something like "to accomplish" or "to bring to a satisfying completion". "我做不了" would mean "I'm unable to do it" (opposite of 做得了). The Cantonese counterpart to this would be 到 (in second tone): "我做唔到". A literal translation of 不得了 into Cantonese would thus be 唔得到, but since it's a fixed expression it's not translated.
On its own, 不得了 means "great" or "impressive" (similar to 犀利). 嘩, 你真係不得了喎! I don't think it can be coupled with other intensifiers, so you don't say "佢好不得了".
Finally, 不得了 can also mean "terrible", at least in Mandarin/written Chinese: "如果那樣做, 那可不得了". I'm not sure if I've heard it used like this in spoken Cantonese, though.
EDITed for typo. wrote 唔, meant 咗.