Language and Dialect
Dialects should be mutually intelligible forms of the same language... but they often aren't.
Whether forms of speech are called languages or dialects is often a question of culture, politics, and power. The powerful speak languages, the relatively powerless, dialects.
Serbian and Croatian are mutually intelligible but are considered separate languages for entirely political reasons.
African languages are often called dialects, even though they are mutually unintelligible and more different from each other than European languages are.
French, Italian, and Spanish are never spoken of as dialects of Latin, but Spanish and Italian have more than 80% lexical overlap. Written Russian, Ukrainian, and Belarusian are about 85% mutually intelligible. Whether these are dialects or languages is clearly a question of history, power, and politics.