Question

1. Look at the Canadian national flag.
2. Look at the national flag of Canada.

3. This is the Chinese national flag.
4. This is the national flag of China.

5. This is American national flag.
6. This is the national flag of America.

(Are they all grammatical? Do we have other expressions for those?)

Answer 1

All are fine. The sentences can also be said without the word "national" in it.

Answer 2

Those are all correct.

I can't think of any other ways of saying these statements.