If you aren't Nigerian and you speak english, there is a high chance that I will not understand hat you said quickly not because you aren't saying the right words but because accent is involved. Asides people from different countries, it has been seen that accent can also be an indicator of class, Ethnicity as it is in Nigeria where you can tell a person is Hausa, Igbo, or Yoruba from their accent when speaking english, and it can also indicate educational status. While Accents are specific to different people from different phase of the world, some people just pick up accents from places they haven't been before or have affiliation to.
This was a case of Karen Butler, an Oregonian woman who went to the dentist to replace her teeth with dentures. While the surgery was simple and straightforward, she woke up from her anesthesia feeling numbness and dryness in her mouth but that wouldn't bother her since she just had a surgery but then she tried to speak, she began to speak oddly and the dentist wrote it off as the denture being behind the wacky talk, and she was going to be fine.
Days past and she had an accent that would not go. Her English accent was a combination of Romanian, Irish, other European language. She visited the neurologist who diagnosed her with Foreign Accent Syndrome (FAS). This syndrome impacts the phonetic system of a person. People with Foreign Accent Syndrome (FAS) exhibit symptoms which includes removing or substituting sounds they wouldn't normally change. They can have problem with sounds that has to do with tapping the tongue behind the teeth.
Foreign Accent Syndrome can be Neurogenic, Psychogenic, or mixed and their difference is as a result of causation. In the case of neurogenic Foreign Accent Syndrome, it is caused by a damage to the central nervous system as a result of conditions like stroke, Aneurysm, Traumatic Brain Injuries, and Multiple Sclerosis. There haven't been lots of cases that have been properly diagnosed so there is still little known about this case. It is said to be a malfunction within the Broca's area which is along with the Wernicke's area which are responsible for speech formation. In Neurogenic FAS, while the speech language is stored properly in the wernicke's area, but the motor program in the broca's area when pronouncing the word is damaged causing slanted language.
In the case of psychogenic Foreign Accent Syndrome is caused by underlying psychogenic problems such as conversion disorder. It is said to be associated with people experiencing blindness, paralysis and other symptoms affecting the nervous system that cannot be explained by physical injury or illness. The mixed type of FAS originates from a psychological impact or neurogenic damage.
Since the causes can vary, treatment needs to be tailored to the individual but then speech and behavioral therapy are he primary method for treatment when dealing with this type of condition. If there are cases of underlying issues, the treatment would focus on the underlying cause.
Post Reference
https://www.npr.org/sections/health-shots/2011/06/01/136824428/a-curious-case-of-foreign-accent-syndrome
https://www.sciencedirect.com/science/article/abs/pii/S0911604410000576
https://www.healthline.com/health/foreign-accent-syndrome#causes
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4746356/
https://www.sciencedirect.com/science/article/abs/pii/S0911604410000576
https://www.healthline.com/health/foreign-accent-syndrome#treatment
Image Reference
Image 1 || Deviant art || Cuddling Accents
Image 2 || Wikimedia Common || Broca's Area vs. Wernicke's Area