Languages - how there are so many different ones spoken by one animal (makes me wonder if animals have "foreign" languages, dialects etc - there's apparently evidence they do) how language developed and which countries ended up not only speaking different languages but even completely different symbols for those languages in the form of alphabets and accents et
Have a look at David Crystals works, some fascinating stuff.
A couple of things to ponder, on mainland Europe the accents / dialects spoken in boarder places are understood by the people on the other side of the boarder, this goins in continuum of about 10 miles, so if you are French but on the Belgian boarder you will understand the Flemish spoken just over the boarder.
You can go in a continuum of 'understanding' from Calais to Russia with people in the same 10 miles understanding but not understanding people further away.
Nicaraguan sign language. Nic aragua did not have many deaf people until they were sanctioned by the US in the 1970s/1980s. One thing they could not get was modern antibiotics, they had access to older ones but these have the side effect of causing deafness.
Nicaragua started a school for deaf children. Once in contact with each other the developed a language from scratch with all the grammatical features of other languages.
Oh and a final thought, there is an argument that English is not a true language but a Creole. It steals words and grammar from other languages and takes the simplest forms eg we usually add an s to make a plural, this is simpler to understand that other endings which is why it dominates English but we still have remnants from Older languages spoken in the UK eg plurals such as children, men, women, radii.
Some of our dialect words appear as real words in other languages, eg the Yorkshire word, "lake" meaning play is probably a hangover from the Vikings, leka is play in Swedish.