"Necesito decirle algo."
Translation:I need to tell her something.
Yes, the "le" attached to the verb "decir" is the indirect object pronoun, which can be him or her or it. Its ambiguous, so could also add "a ella" or "a él" to the end of the sentence to clarify her or him. The "le" could also be put before the string of verbs, like "le puedo decir algo" instead of being attached to the infinitive verb.
Robert, your comment is interesting and makes me curious. I still believe that for DL's translation "them" would not be correct. I don't believe Spanish uses a singular pronoun to represent an unknown "them". Perhaps we have a native speaker that could confirm or deny my opinion.
in your example, even in common US speech .... '...an applicant, have you decided who they are', 'have you decided who it is', 'have you decided who he/she is' are social choices to avoid gender bias. The oddity/error will be number, de-personalization, or social...i think US frequency for those over 25 is probably the '...an applicant, have you decided who it is'
I misunderstood your question. While "have to" and "need" have the same meaning, the sentence in question is asking for a translation of "necesito." The only acceptable answer for that would be "need."
In everyday practice though, you can change this sentence to "tengo que dicerle algo" and it would mean the same thing.