天真 = innocent, why 'sky' or better 'heaven'? 'Heavenly real'. It makes sense, but translation doesn't work. Concept is hard to be translated into another language.
It’s not just a translation issue, but that the composition is just archaic. The constituents had real meaning a long time ago when they became a word, but today you just understand 天真 as its own word and move on. If you didn’t know that word but understood 天 and 真 you wouldn’t be able to figure out the word’s meaning.
The same is true with many English words as well actually: many of them started out as composites that were meaningful in the last but today are not. For example, “understand.”
Yes, 'heaven' is more precise. To reflect 'heaven' usually requires a deeper grasp of the language to twist it a little bit closer for its real meaning.
And your point is great - the concept is hard to be translated into another language. For example, there are so many Buddhism concepts which is very hard to translate.
The same is true with many English words as well actually: many of them started out as composites that were meaningful in the last but today are not. For example, “understand.”