clari_mis - Well, it seems to me that in this case everything depends on the situation.

benni_sti - Well, if, for example, a guy introduced himself as a Spaniard to attract a girl, then this is wrong.

rudolf_w - I agree that everything depends on the situation here. When a person deceives you about his own nationality for subsequent benefit, it is wrong.
For example, if you come to study at a cool Spanish language school, you were promised a Spanish teacher. You will be unhappy if he turns out to be a Mongol in the end. Therefore, it is more reliable to learn the language yourself with the help of dictionaries and textbooks.
I recently bought a good Spanish dictionary for myself in an online store — https://www.amazon.com/Spanish-Frequency-Dictionary-Essential-Vocabulary/dp/9492637200. It's worth giving money for this.