If it is a proper noun, it must be capitalized.
If it is an adjective derived from a proper noun, it should retain its capitalization, according to this Wikipedia entry:
In English, adjectives derived from proper nouns (except the names of characters in fictional works) usually retain their capitalization
– e.g. a Christian church, Canadian whisky, a Shakespearean sonnet, but not a quixotic mission, malapropism, holmesian nor pecksniffian.
Where the original capital is no longer at the beginning of the word, usage varies: anti-Christian, but Presocratic or Pre-Socratic or presocratic (not preSocratic).
The "usually" might explain why you sometimes see "english" without any capitalization. The only case of "english" as a common noun would be in the context of pool, billiards or bowling games, as described by Wiktionary:
- (US) Spinning or rotary motion given to a ball around the vertical axis, as in billiards or bowling.
You can't hit it directly, but maybe if you give it some english.