"一丝淡淡的笑容" can be translated into English as "a faint, subtle smile" or "a hint of a gentle smile".
Both translations capture the essence of a slight, barely noticeable yet warm smile:
"A faint, subtle smile" emphasizes the softness and understated nature of the expression.
"A hint of a gentle smile" conveys the idea of a barely visible yet kind smile.
Choose the phrase that best fits the context or tone you wish to convey!