Rinse rice under cold water until mostly clear. Drain well.
Combine rinsed rice, coconut milk, water, salt, and sugar in a saucepan. Add lemongrass and lime leaf if using. Bring to a boil over medium-high heat, stirring once. Reduce heat to lowest setting, cover tightly, and cook for 15 minutes without lifting the lid.
Remove from heat and rest, covered, for 5 minutes.
Discard lemongrass and lime leaf. Fluff rice gently with a fork.
Dice mangoes and toss with lime juice, lime zest, oil, cilantro, mint, and green onions in a small bowl.
Fold mango and herb mixture gently through the warm fluffed rice. Transfer to a serving bowl, scatter toppings, and serve with lime wedges.