Season both sides of the chicken breasts with salt and black pepper.
Place the seasoned chicken breasts in the slow cooker in a single layer.
In a mixing bowl, combine 2 cups diced mango, 1/2 cup diced red bell pepper, 1/4 cup chopped green onions, 1/4 cup chopped cilantro, and 2 tablespoons olive oil; toss to combine.
Spoon the mango–pepper mixture evenly over the chicken in the slow cooker.
Cook on high for 4 hours or low for 6 hours, until the chicken is fully cooked and tender.
Remove the chicken to a cutting board or bowl and shred with two forks or in a stand mixer on low with the paddle attachment.
Toss the shredded chicken with any juices from the slow cooker and additional mango–pepper mixture if desired.
To make the mango and strawberry salsa, combine 2 cups diced mango, 1/2 lb diced strawberries, 1/3 cup chopped cilantro, 3–4 tablespoons fresh lime juice, and the finely chopped jalapeño (if using); toss to combine.
Serve the shredded chicken in warmed tortillas and top with the mango–strawberry salsa.