@fBoyle-- Those are some great questions. Of course, not all Muslims know Arabic but all Muslims recite prayers in Arabic. These are prayers taken from the Qur'an and the hadith. Muslims memorize them and recite them. Even if Arabic is not their native language, most Muslims read Arabic Qur'an and/or Qur'an in their own language and know what the prayers mean. Moreover, in most Muslim countries, Qur'an classes are offered where children and adults can learn to read and write in Arabic so that they can recite the Qur'an.
I've never seen Islamic calligraphy in a language other than Arabic but I could be wrong. Most people purchase Arabic religious calligraphy and hang it up in their homes and workplaces for blessings. For example, we have one in our home which says "bismillahirahmanirahim." It means "in the name of Allah, the most merciful, the most compassionate." It is a blessing and good luck to start everything by reciting this first.