Why is Japanese so hard to translate ?

If you mean translation by using Google Translate then the reason are

1. Because Japan and English has different grammar construction. English is SVO and Japan is SOV.
2. Different interpretation of paragraphs context by Google Translate system. Google Translate Not so good for translation Japanese long words or long paragraphs but still dependable for a single word or very short/very simple paragraphs. Recent slang words and new idioms also take in place.
3. So at least you need some basic knowledge about Japanese language when using Google Translate.
4. This problem also happened to other languages have different grammars construction and different contexts interpretation system with English. These things make English translation from Japanese on Google Translate appears “error" in your eyes.

*Hinako Hasegawa from Tada-kun wa Koi wo Shinai