This is a short (but important!) post. When writing in English, it is important to always capitalize the words for languages and nationalities.
Chen is Chinese and speaks Mandarin, Cantonese, and English, while his co-founder is Brazilian and speaks Portuguese and English.
This mistake is especially common with the word English. As English is a proper noun, it must always be capitalized. No exceptions!
What if There is More Than One Word?
If the language or nationality has more than one word, you should always capitalize both of them. For example:
Li is Chinese-American.
Many new tech startups are selling to Mandarin Chinese speakers.
Florian speaks Swiss German fluently.