Why uppercase and lowercase?

The conventions of using uppercase and lowercase letters evolved over centuries. The terms themselves relate to printing and typesetting.

Printing presses had manually set letters (movable type).

Small letters were used most often, so they were kept in the lower, easier-to-access case. Large (capital) letters were kept in uppercase. 

Script

Capitalisation is a feature of script, not language. All languages using Latin script have uppercase and lowercase. Usage conventions have evolved over centuries and differ between languages.

For example, French uses uppercase less than English. English capitalises days of the week and languages.