The numerous contributions above have not pointed out one key difference between the English and German languages.
In English, the general rule (with few exceptions) is that you can use the adjective denoting nationality also for referring to a person/persons that come from this country. So you say:
He is English (meaning he is a person from(born in) England.
He is Italien (same for Italy).
In German, you
derive a noun from the adjective denoting nationality
or from the name of the country, with no intelligible rule to distinguish between these possibilities.
Look e.g into the following weirdness:
The Irisch - der Ire (not: der Irländer), die Irin
The Islandish - der Isländer (not: der Ise), die Isländerin
The Finnish - der Finne (not: der Finnländer), die Finnin
The English - der Engländer (not: Der Enge), die Engländerin
The German - der Deutsche (not: der Deutschländer), die Deutsche
(There is an odd sausage company that was crazy enough to call one of their sausage brands "die Deutschländer". They should have called their sausage "die Sprachmörderin").
The Greek - der Grieche (not: der Griechenländer), die Griechin.
The Italian - der Italiener (not: Der Italier, Italer (these are words used to denote ancient inhabitants of Italy), also not: der Italienische), die Italienerin.
Plus oddities (by the dozen!) like:
The Portuguese - der Portugiese (not: der Portugaler, der Potugiesische), die Portugiesin
Er ist deutsch (e.g. Der Satz ist leicht zu lesen - er ist deutsch. Meaning that it (the sentence) is German (adjective, capitalized in English, but not in German).
Er ist (ein) Deutscher (Meaning: He is (a) German in the sense "of German nationality/birth/ancestry").
"Deutscher" is now a noun ("Substantiv") falling into the 22-odd rules how to make a declination table for nouns (very good in this respect: Wahrig Wörterbuch (dictionary) or www.canoo.net).
In diesem Sinne:
Meine Mutter ist (eine) Deutsche. Correct.
Meine Mutter ist deutsch. Wrong (!). At least not used.
"Meine Mutter" ist Deutsch/deutsch (Meaning that the text is written in the German language rather than in the English/Russian/Chinese language. Quotation marks mandatory!)
Mein Vater ist Deutsch. No way. see "Meine Mutter" above.
Mein Vater ist ein Deutsch. Also no way. Mein Vater ist (ein) Deutscher. Nothing else. Surely. Native speaker. Yessir. Jawohl!
Nicht alle Deutschen trinken Tee. ("alle Deutsche" sounds VERY awkward to me, for an intricate discussion cf. the "Zwiebelfisch" article of SPIEGEL Online).
Does this add to your (John's) feeling for the German language?
For further reading: Mark Twain, "The Awful German Language",
http://www.crossmyt.com/hc/linghebr/awfgrmlg.html .