My tongue has a yellow film on it for 5 months now. The doctor gave me a medicine to treat a yeast infection but it didn't help. What else could it be?

Does it bother you? What i mean is besides the fact that you have a yellow film does it bother you at all? If you're not having pain irritation burning or any other symptoms then forget about it. It is amazing how different people's tongues can look at various times. Frequently what you eat and drink can affect how your tongue looks. Acids can especially alter the tongues appearance.
Reflux. If there is no yeast then there could be the result of reflux from your stomach or a vitamin deficiency. B complex is safe to take to see if it improves the appearance of your tonque. If there is reflucx present it could also affect your larynx with a change in voice or a burning sensation in your mouth. Medications for reflux are readily available. Discuss with your doctor.
You need to see PC . It would be best to see your primary care doctor. Lab tasts and exam should lead to the correct diagnosis.