If the qur'an was sent for all mankind, then why in order to understand the true meaning and depth of the Qur'an, one has to be able to understand and speak Arabic?
Allaah, the most high burdens not a person he/she could not handle. Allaah swt gave us the capability to think and learn, therefore to learn arabic is not something impossible to do. Also, translations are not %100 accurate as there are words from a language that doesn't have the exact match in another language, therefore, they use the closest one to it instead. So to rely mainly on the translation itself which is only created by man, can result to misguidance and misinterpretation.