What baud rate can be supported by this internal oscillator?
You should read the datasheet of the STM32 MCU. The clock divider for the baud rate must be able to produce 38.4 kbps. 48 MHz divided by 1250 will be 38.4 kHz. Using a divider of 1280 = 5 * 256 is possible, the error of 2,3 % ist acceptable for asynchronus serial transmission of 8 databits and 1 start bit, a stop bit and a parity bit. A divider 1248 = 39 * 32 would be better. Look for the possible clock divider values. The internal 48 MHz oscillator should be better than +/- 1 % error.