The tense of the Spanish is present, but the English seems to be in the past?
You COULD interpret the English as the present tense, but it could also be past tense and to me that’s more obvious. I think “I’m letting them go” is a clearer English translation.
Yes. Although it is possible to say “I let them go” in English to mean present tense, it is unusual, and you’d usually only hear it used to describe an ongoing or habitual action, for example if you said: “I always let them go.” or something like that. With no additional context or qualifier I think it is most reasonable to interpret the English sentence as past tense. You could however hear the English sentence used in a specific context, to mean present tense. For example imagine this exchange:
“Well, what do you do with them?”
“I let them go.”
In this case it is clear from context that it is present tense, and describing a habitual action.
Language is such a tricky and subtle thing sometimes!