Heat oil in a large skillet over medium heat. Cook garlic, stirring occasionally, until golden brown, about 2 minutes. Add red pepper flakes and half of Swiss chard, season with salt and pepper, and cook, tossing often, until wilted, about 4 minutes. Add lemon juice and remaining chard and cook, tossing, just until all chard is wilted, about 1 minute; season with salt and pepper.