The other answers are correct, but the article can be omitted in the plural. Some people think French accents are romantic. Some people think a French accent is romantic. (Both are ok and most people would perceive the two sentences as having a similar meaning. )

Yes, use an article. -She has an Indian accent. -He has a British accent. -How to practice an American accent. -The British accent is harder to learn than the American accent. In the last example, I use “the” because it is an abstract accent (i.e. the British accent in general), not a particular accent (i.e. I have an American accent). More examples: I like how the British accent sounds. I met someone yesterday who has a British accent. The American accent is easier for me to understand, because I have an American accent. You wouldn’t say “she speaks the British accent,” you would say “she speaks with a British accent.” Here, the accent we are talking about is hers, so it is a particular accent.

"Accent" is a countable noun, so it takes a determiner (a, an, the).

Yes you always need an article with the word "accent". Use of "the" can make it more specific. She has an American accent. She has the accent of a hard headed New Yorker. She has the accent of a Southern Belle. The accent he has is not quite native. He has a slight French accent. A Russian accent is very sexy. = all Russian accents are very sexy The French accent is very sexy. = That French accent s/he has is very sexy. " She speaks the British accent." is not correct. "She speaks with a British accent." - correct. "She has a British accent" - correct. "Her accent is British" - correct. "The accent she has is British." ok if someone has asked what accent she has or what her accent is.

You must use an article with "accent". In the examples you give, the indefinite article is correct. The sentences with no article are incorrect. However, we can also use the definite article with "accent". We use the definite article when we are talking about a specific accent (for example a country's accent) and we want to emphasize that this accent is unique and different from other accents. This is common in sentences when the accent is the grammatical subject of the sentence. For example: "The Northern Irish accent is my favourite" "The Yorkshire accent is very musical" "The Australian accent is so relaxing" But it can also be used when the accent is the object of the sentence but we are emphasizing how the accent is unique or different. For example: "I love the French accent" "Is the New York accent different to the Californian accent?" "It's difficult to imitate the New Zealand accent" Therefore from your examples we could also say: "How to practice the American accent." "The British accent is harder to learn than the American accent." (but your examples with the indefinite article are also correct) ["She speaks the British accent" is not correct, but we can say "She speaks with a British accent". In this sentence, because the accent is not the subject of the sentence, and I am not emphasizing how unique or different the British accent is, I use "a" not "the"].