Did the Messenger (peace be upon him) master languages other than Arabic (such as French and English), or is the intent that he only mastered Arabic dialects?

The statement that the Prophet, peace and blessings be upon him, encouraged learning languages is inaccurate. The correct account is that he commanded Zayd ibn Thabit to learn Syriac because he needed someone to translate the writings of the Jews for him. By analogy, this extends to learning any language that Muslims need for their dealings and relationships. The Prophet, peace and blessings be upon him, did not know languages; rather, he was unlettered, neither reading nor writing. Had he known languages, he would not have asked Zayd to learn Syriac.