Not quite. While "Chinese" and "Portuguese" are used for both the demonym and the adjective, they are typically used with articles like "a" when referring to people in specific contexts. So the correct sentences would be: "I saw a Chinese person." and "I met a Portuguese person." In general, you should include "person" to make the sentences clearer.

A Portuguese person is simply referred to as "Portuguese," and a Chinese person is referred to as "Chinese." In both cases, the term serves as both the demonym and the adjective.