Unicode: Using the standard
By coding characters on several bytes, Unicode now allows to write most languages in their own writing system, as well as various transcription or translitteration systems. All softwares still don't implement it, but it is the case of Mediawiki and more or less of Word. So we can write composite texts including words in different languages.
With a Mac, you need to go to "Display Visualisor of characters" (a scrolling menu under the flag above right). You then chose your set of characters. By clicking twice on a character, it is inserted.
Another solution is offered by the French site Lexilogos, which proposes keyboards in many languages. Above each key, the equivalents in translitterated / original characters appear, which is most practical. You only have to click once on a character, and the word appears in a window. For the Arabic language, the letter form adapts itself to its environment (beginning, middle, end). You then simply have to copy the word or the phrase into the desired software. To be noted: Mediawiki also manages the form of Arabic letters, whereas Word is unable to do it.