If you’ve ever wondered just how much mutual intelligibility there between different languages, the sites I came across today will go some way to satisfying your curiosity.
One site is a Wikipedia page which discusses the degree of mutual intelligibility between a wide range of languages for quite a few different language families. It divides languages into those that are mutually intelligible in both their written and spoken forms, their spoken forms only, or their written forms only.
Then there’s a long discussion on this site about the mutually intelligibility of Slavic languages. There seems to be many different opinions on this topic.
I find that with a fairly good knowledge of French and Spanish, plus a basic knowledge of Portuguese and Italian, I can read and sort of understand just about any Romance language, with the exception of Romanian. When I hear unfamiliar Romance languages spoken, I can get a basic idea of their meaning as well.
My German helps me to understand written Dutch and Afrikaans to some extent, and to understand a bit of their spoken forms. In fact I understand slightly more of them than of German regional dialects such as Bavarian.
My limited knowledge of Czech and Russian helps me understand bits and pieces of other Slavic languages, at least when they’re written.
Knowledge of Irish enables me to understand quite a lot of Scottish Gaelic, and some Manx, though as Manx spelling is so different, it takes a lot of decoding.
I can understand some written Cornish thanks to my Welsh abilities, though hardly any Breton.