"Creo que sí, pero no estoy seguro."
Translation:I think so, but I'm not sure.
This is a fine example of a colloquial translation because the literal translation of "Creo que sí" is "I think that yes." While "I think that yes/Creo que sí" is colloquial Spanish, it is not colloquial English. Hence, the substitution of "so." "I think so" IS colloquial English.