Sindarin is the term given to the written and verbal language of the Sindar. As the language of the lost people, Sindarin is usually only known to those with an interest in the study of the ancient people.
Sindar script
Sindar script appears at first to be a true foreign language, it is actually a substitution cipher and can be easily translated. Sindar script gives a counterpart for each of the 26 letters of the Latin alphabet. Sindar Script is, not surprisingly, found in ancient Sindar texts and ruins. It can also be found on one of the emblems of the Holy Kingdom of Harmonia.
The Sindar hunters, Lorelai, Killey and Zweig each held some measure of ability in translating the script whereas relatively studious persons, such as Jowy Atreides and Alex, could at least recognise the ancient text on sight.
Spoken Sindarin
Compared to being able to read, or at least recognise, Sindar script, the ability to speak Sindar appears to be a much rarer talent. So far, only those exceptionally skilled at investigating the Sindar, such as Luc and Sarah have been shown able to speak the Sindar tongue.