To be honest, I don't think there's a definite answer. It could be a blend of fact and fiction, but without official confirmation, it's hard to tell exactly how much of it is based on true events.
Well, it's hard to say for sure. Sometimes series draw elements from real life but blend them with imagination. You'd have to do some research or check with the creators to know for certain.
One of the main themes is adventure. Many stories in the series involve characters going on exciting journeys, like in 'The Call of the Wild'. Another theme is love, such as in 'Romeo and Juliet'.
I'm not sure specifically as I haven't read it. It could be about lost or forgotten elements of the English language presented in a story form, perhaps things like archaic words, idioms that are no longer commonly used, or old English dialects.
I have no idea who the main characters are without reading the 'lost english series story'. However, it's possible that there are linguists as main characters, as they are often the ones interested in the evolution and loss of language. Or perhaps ordinary people who stumble upon forgotten English words and phrases and their lives get changed because of it.