Why is the word français written with a small letter and eg Deutsch with a capital letter?

Answer

The use of capital letters is based on conventions, agreed rules. In one language one can prescribe that languages ​​be written with a capital letter, in another language not. Thus the grammarians have determined that in French the names of persons (Français) are written with a capital letter, but the adjectives (la langue française) and the names of languages ​​with a small letter (le français, le néerlandais). In Dutch people write names of languages ​​with a capital letter. This is also a conventional rule. In German, all nouns, including those of languages, are capitalized.

Capital letters can serve several purposes when establishing the rules of written language: to indicate proper names (such as persons, countries and cities), to indicate the beginning of a sentence, to indicate important institutions (Parliament) or to express respect (God, the King).