Why is Russian word “сидеть” (“sidet'” which means “to sit”) so similar with English word “Sydney”?

As others have said: sometimes, coincidence happens.

Sydney was named for Thomas Townshend, 1st Viscount Sydney, and Sidney (surname) derives from Old English sīdan īege, “at the wide island”, i.e. Water-meadow.

Old English sid means “wide, extensive, broad”, and is the ancestor of Modern English side. Wiktionary tells me it comes from Germanic *sīdaz, which comes from Proto-Indo-European *seh₁(i)- ‎(“to send, throw, drop, sow, deposit”).

сидеть, like sit, comes from Proto-Indo-European *sed-, via Proto-Germanic *sitjaną.

So your question, OP, is actually why are *seh₁-d and *sedso similar. And the answer still is, sometimes, coincidence happens.

Is there a tendency for languages to gain or lose complex clusters over time?

I’m saying the same thing as JJ Hantsch, but with different emphasis.

Is there a tendency for languages to gain or lose complex clusters over time? Both, but with different causes. Languages lose complex clusters through various processes of phonotactic simplification. Languages gain complex clusters through dropping vowels [EDIT: or adding consonants for ease of pronunciation: Epenthesis].

The gaining of clusters is pretty random, because unaccented vowels can drop out anywhere. Northern Greek dropped all unstressed high vowels, and all of a sudden had to deal with clusters like piðo > pðo “I jump” or mikros > mkros “small”. (At least some variants have dealt with it by epenthesis: imkros. Because mkros NCCVC is an unusual cluster, at least with the NC not being homorganic.)

The simplifications are not fully predictable: for example, Tsakonian has pummelled Greek clusters into CV, with unusual shifts like st > tʰ and tr > tʃ. But yes, I think we can say there is a tendency towards CV in language; it’s just a gradual, random, piecemeal, and easily disrupted tendency—to the extent that it’s not much of a tendency.

Do syllable templates change, as in languages going from CCCVCCC to CV? They can; Tsakonian did. And they can go the other way too, as Northern Greek did.

That’s why dialectology is awesome btw. You get to see multiple outcomes from the single starting point.

Icelandic (language): What is flámæli?

What Lyonel said. I’m away from my references 🙁 , but see North American Icelandic.

The story is that Icelanders noticed the merger in the 1920s, stigmatised it as “fisherman’s language”, and got rid of it successfully (although the link says that the e/ö merger is still around). In North America, of course, no prescription, so flámæli has kept going, which what the linked book is about.

The importance of flámæli, which is why I keep mentioning it here and misspelling it, is that language change can be reversed through prescription, but you need special circumstances; and Icelandic is as “special” as it gets (small community, universal literacy). And given that the e/ö merger shows up in Reykjavik teens 60 years later… I guess the success was only temporary.