"He who has overcome his fears will truly be free."
Simple: Someone who does not let fear stop him will really be free.
"I count him braver who overcomes his desires than him who overcomes his enemies."
Simple: Someone who does not let his desires stop him seems to me to have a stronger mind (less fear) than someone who does not let his enemies stop him.
"In all things of nature there is something of the marvelous."
Simple: In every thing of nature, there is something to make a person wonder and feel good.
"Misfortune shows those who are not really friends."
Simple: When a person has bad luck, the person finds out which of their friends are not really friends.
"How many a dispute could have been deflated into a single paragraph if the disputants had dared to define their terms."
Simple: It is a large number of arguments that the people arguing could have made smaller into just a small number of sentences, if they had not been afraid to take the words they were using and explain the meaning of each word.