Plural of s = s'es?

There are other languages with their own alphabet, but I don't know how many languages have alphabets. Perhaps another member can enlighten me. Mandarin has no alphabet.

Yes, I understand that.  But this usage would be very rare and most likely not in shortform.

We don't use Mandarin to refer to the Chinese language.  Mandarin is one spoken dialect of the Chinese language, albeit the official one and is taught all over the country.  It refers to the spoken language, not the writing.  The PinYin system commonly used for Mandarin is a way to transliterate the sound.

I suppose most languages are based on alphabetical systems.  Chinese and only a handful are exceptions.

The Chinese language has many dialects.  We all share the same set of characters but these characters are pronounced differently in different dialects.  Same characters different pronunciations.  We don't say the characters belong to any one particular dialect.  They are just Chinese characters.  You can't say Mandarin characters or Cantonese characters because these terms do not make sense.  For example, the numbers one, two, three are written as one, two, and three horizontal strokes respectively in Chinese "一 二 三" (can you see these characters?) They are pronounced as yi1 er4 san1 in Mandarin and jat7 ji6 sam1 in Cantonese.  These are not exact transliteration though.  Chinese is a tonal language and the numbers are used to indicate the tone.  Mandarin has four tones while Cantonese has nine (some say six depending on which method you use to indicate the tones).  So, everyone can read the same Chinese book but they read them differently.

Actually, the apostrophe shouldn't be placed there. It should merely be PSs - similar to the "1980s" not "1980's"
Also - the s's looks kinda weird...maybe it's meant to be like s' or something o__O; for e.g. it's meant to be "James' wallet", as opposed to "James's wallet" <--except American spelling often utilises this form