Not sure if it's widely available in English. But sometimes, fan translations or limited editions might exist. You could search on specific literary forums or communities to see if there are any.
The slight smile in the novel is often used to show the character's hidden feelings. For instance, when the male lead first notices the female lead's slight smile, it might be a sign of her initial interest, which is a very subtle way of expressing emotions.
The characters in 'A Slight Smile Is Very Charming' are very vivid. For example, the female lead has a unique charm. Her slight smile can be seen as a sign of her intelligence and confidence, which makes the whole story more attractive.
One reason could be the relatable characters. In 'A Slight Smile is Very Charming' fanfiction, the characters often have complex personalities. Their interactions, like the shy glances or the witty banter, draw readers in. Another aspect is the world - building. Even though it's fanfiction, it might expand on the original setting in interesting ways, creating a more immersive experience for the readers.