In English, words that describe nationalities or regions are always written with a capital letter. We write about Spanish music, Italian food, and German cars. Because this rule is so heavily drilled into English speakers, it is very common to make the exact same assumption when writing in German.
However, German handles these words differently. When a nationality, region, or language acts as an adjective describing a noun, it is written entirely in lowercase. You will frequently see words like deutsch, englisch, and französisch starting with a small letter.
The rule is incredibly straightforward once you break your English habits. Whenever you use a word of geographic origin to describe a noun, treat it like any other regular adjective. Words like deutsch, italienisch, and französisch take standard adjective endings based on the noun they describe, and they never take a capital letter unless they happen to be the very first word in a sentence.
Er fährt ein deutsches Auto.
(He drives a German car.)
Ich liebe italienisches Essen.
(I love Italian food.)
Das ist ein französischer Wein.
(That is a French wine.)
Languages and regions
This lowercase rule covers all adjectives describing geographic origin or language. Whether you are talking about specific languages and countries like englisch and spanisch, or broader regional adjectives like europäisch and chinesisch, you must keep the first letter lowercase.
Sie lernt spanische Vokabeln.
(She is learning Spanish vocabulary.)
Wir schauen einen englischen Film.
(We are watching an English movie.)
Chinesischer Tee schmeckt sehr gut.
(Chinese tea tastes very good.)
Die europäische Geschichte ist faszinierend.
(European history is fascinating.)
Watch out
There are two important special cases to keep in mind. First, if a geographic prefix is a capitalized abbreviation, that prefix keeps its capital letters, but the adjective itself stays lowercase. You can see this clearly in US-amerikanisch, which contrasts slightly with the purely lowercase amerikanisch.
Er mag amerikanische Musik.
(He likes American music.)
Mein Kollege ist US-amerikanischer Staatsbürger.
(My colleague is a US-American citizen.)
Second, remember that this lowercase rule only applies to adjectives. If you are talking about the language itself as a noun, or a person from that country as a noun, you must capitalize it according to regular German noun rules. For instance, the adjective in ein deutsches Buch (a German book) is lowercase, but the language noun in ich spreche Deutsch (I speak German) is capitalized.
Summary
Remember that nationality and language adjectives are just normal adjectives in German. As long as they are describing a noun rather than acting as a noun themselves, you should always write them in lowercase.