Have a look at David Crystals works, some fascinating stuff.
I have, as I say I've studied this but so much of it is theory as languages developed so long ago we'll never REALLY Know EXACTLY eg why one alphabet is one way and another totally different. Why even with the same alphabet certain words, letters etc are pronounced and used differently in different languages even when they're closely related. So much of it developed organically it's hard to trace exact roots.
I am trained in BSL so I find your comments on Nicaraguan sign language fascinating. When learning I was also surprised to learn how very different American Sign Language is to BSL.
there is an argument that English is not a true language but a Creole. It steals words and grammar from other languages
Yes I said pretty much the same upthread. I do find it very amusing and ill informed when people online especially on mn correct other people's spelling and grammar and such and even denigrate their use of English when the other poster is eg from USA usually at some point saying something like "use proper English!" When of course there's really no such thing!
When I lived overseas as an adult (I'd lived there before as a pre-verbal/barely verbal child but don't really remember) I was worried about the language barrier as at that time I only had some schoolgirl french that I'd not used in years! This was also prior to my studying the English language and so I knew very little about European language history.
As it turned out my knowledge of Scots Gaelic came in very handy! That will have confused a few of you reading!
Turns out they're all Germanic languages and I was able to fairly quickly pick up enough to get by just from my previous knowledge of Scots Gaelic, I learned more the longer I lived there of course as immersion and necessity REALLY helps! Early on I got on the wrong bloody bus! To find my way to getting the RIGHT bus home involved using a mish mash of the few Dutch words I'd picked up already, a few french, and a few Gaelic! Fuck knows what people witnessing that conversation would have thought!
Being a Scot I also found it easier to pronounce certain words too, as I could pronounce guttural sounds eg ch as in loch or rhotic r etc
This actually caused me problems as my pronunciation made it seem as if I were a fluent speaker at times and then the other person would be like "great I can speak as fast and as dialectically as I like!" And blether away at me as I looked  and then went "sorry can you say that again slower" which confused them!
I fear you and I @sashh Could blether on about this all day and then some!
Where my fascination REALLY lies and I'd LOVE to indulge in studying properly is idioms and proverbs etc
Having lived all over the Uk, Western Europe and having friends and family all over the world I LOVE when someone tells me about a new idiom or similar that they've discovered and the meaning and history behind them. They are the very definition of language being culture led AND vice versa
Just think of the bizarre ones in the English language alone
Raining cats and dogs
Fish out of water
Fit as a fiddle etc.
Totally nonsensical!
When I did my first trip to France as part of a school exchange programme (don't think you'd get away with THAT these days, living in a strange families home, likely no background check and a language barrier!) and was amazed to learn that other languages had idioms etc too.
The daughter I was matched with was a moody mare and while fighting with her older sister who I got along great with she said something I couldn't make head or tail of and thought I'd simply made a mistake. I ended up asking one of the other exchange students when we next met and they laughed and said no mistake I had it right it was an idiom. Something about running on her beans iirc which meant she was driving her nuts/making her angry.