Configureable values for flesh kincaid tests becuase of different result depending on language

Great idea, thanks for proposing it. I'm going to think through the best way to make this configurable for any language. For the moment, we can start with the known formulas for English, Spanish, and German and those three can be defaults in the settings screen. In the future, we can add additional languages once we know the formulas. I found this: https://www.spanishreadability.com/spanish-flesch-kincaid-grade-level that talks about the same Spanish formula, but would be great to find other formulae for other languages too.