Did you know that each country has a different sign language? And they are not similar, even if the spoken languages of the respective countries sound similar. From Wikipedia:

On the whole, sign languages are independent of oral languages and follow their own paths of development. For example, British Sign Language and American Sign Language are quite different and mutually unintelligible, even though the hearing people of Britain and America share the same oral language. [...] Similarly, countries which use a single oral language throughout may have two or more sign languages; whereas an area that contains more than one oral language might use only one sign language.

To me this seems very irrational. I don't understand the details, but from short reading of Wikipedia it seems that many sign languages have a common origin, and that for each country there was historically an institution defining and teaching the language; if there were more such institutions in the country, sometimes there were more languages. Why? If the sign language is so different from the spoken language, what's the point of having a different sign language everywhere? I think it would be much better to keep the languages synchronized, so the deaf people would at least have an advantage of easy international communication.

(Once I thought about learning a sign language, just of curiosity, but this fact made a "trivial" inconvenience: which sign language should I learn? Slovakian? According to Wikipedia, it's not even similar to Czech sign language, despite similar languages and shared history. American? Possibly most users. International?)