I think the two translations "We are apt to make mistakes." and "We're apt to make mistakes." are not really correct as translations of the German sentence.

The translation "It is normal to make mistakes." seems correct.

The German sentence doesn't reference "we" and thus it could be used in a broader context, such as if talking about another person.

Also, "We are apt..." doesn't mean the exact same thing, it could be seen as meaning "We are inclined..." or "We have a tendency..." which, while it relates to the mistakes being "normal", I think it communicates something different. The statement about it being "normal" to make mistakes seems to have a broader scope, whereas "We are apt" could be used in a more limited context, for example, if you're talking about a "we" who for whatever reason has a greater than normal inclination to make mistakes, and contrasting it with other people who might not.

For example, if the context were like: "(Because we are outsiders in this culture), we are apt to make mistakes (when judging interactions of people in this culture.)" In this case, the mistakes might not be "normal", i.e. they might seem unnatural or unexpected to people in the culture.