I would think in Senegal, if there are so many mothers who bf until 12 months, then you think it is normal and do so. You will know how to do it, when and how to mix it with the solid foods.
When I had DS in 1991, follow on milk was a new thing, and the idea that you needed formula beyond 6 months was a new thing.
Both my parents (born just before the beginning of the second world war) were bottle fed and it seemed the norm. One GM used to tell me about the cow and gate milk, and the banana shaped bottles. I think there had been a lot of change in the milk available and equipment and sterilising, and that combined with the problems of getting extra food in war time, it was quite the thing. Also, it was probably easier to get the baby milk from the clinic than extra food for the mother on rations. People did not have lots of spare money either. DFIL, born earlier in the 1930s, on a farm, had watered down cows milk (dairy farm, of course). They were quite set in the way of feeding every 4 hours (3 hours up to 6 weeks) to make sure that their babies were being properly fed, and babies had routines so that the rest of the housework could be done.
My own mother bf (1960s and 70s), then moved onto bottles (6 weeks to 12 weeks). Some of her friends went on until the baby was weaned at about 6 months, when they could have cows milk in a cup. Bedtime bottles usually continued, but bottles were usually dispensed with by 12 months. Some hippy types fed bigger babies, and a lot of people started with bottles. My mother used to work in a childcare environment, so there were lots of posters about breast is best around there.
I fed DS until 8 months and DTDs until 10 months. There were all the problems of going out etc which I never felt comfortable about (lots in the press of people having to feed DC in toilets etc) DS fed badly after about 4months, would not have bottles, did not like solids and I was desperate for some sleep and normality. We finally got him to take some formula at 7 months in a cup. DTDs were a dream and I would have fed them longer only I was underweight by 6 months.
I found it very confusing as to what to do once they were taking solid food; books were directed at bottle feeding every 4 hours. Anything bf related was an afterthought and usually written for people who were totally demand bf, which I was not doing with twins. BLW sounds a good thing, and I would probably have given it a go if I had known about it, and the HV was not telling me I should give 12 week old 1 month prem twins babyrice.
or that DS should have been on 3 meals a day at 20 weeks.