Why upper and lower case?

The conventions of using upper and lower case 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 frequently and so were kept in the lower, easier to access case. Large (capital) letters were kept in the upper case. 

Script

Capitalisation is a feature of script rather than language. All languages using Latin script have upper and lower case. The conventions regarding usage have evolved over centuries and differ between those languages.

French, for example, uses uppercase less than English. Days of the week are capitalised in English - as are languages.