Hi..
Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concern...
As per your complain a Brown spot on tongue can occur due to a few common causes to a malignant one like
Melanoma..
It can occur due to a few common causes like burning with hot foods,
chemical burn, accumulation of debris over the papillae, bacterial and fungal overgrowth while at times it can be due to Melanoma formation..
If it is present since childhood it can be a normal hyperpigmantation and can be genetic in origin.
It needs a proper clinical evaluation and if something malignant is suspected a
Biopsy can be done to confirm the diagnosis..
Nothing to worry as it can be nothing serious and you can consult an Oral Physician and get evaluated and treatment can be planned as per the diagnosis..
Hope this information helps..
Thanks and regards..
Dr.Honey Nandwani Arora.