Use Overpass Turbo, an interface for the OpenStreetMap database.
You can run queries using Overpass QL
and export results in .geojson format.
Download national borders and parks for 6 Central Africa countries using the following query:
area["ISO3166-1"~"CD|CG|CM|GA|GQ|NG"]->.a;
(
nwr(area.a)[boundary=administrative][admin_level=2];
nwr(area.a)[boundary=national_park];
nwr(area.a)[boundary=protected_area];
);
out geom;
Afterwards, you download the national borders for the remaining African countries (in batches, to fit in API quotas):
AO|BF|BI|BJ|BW|CF|CI|CV|DJ|DZ|EG|ER|ET
GH|GM|GN|GW|KE|KM|LR|LS|LY|MA|MG|ML|MR
MU|MW|MZ|NA|NE|RE|RW|SC|SD|SH|SL|SN|SO
SS|ST|SZ|TD|TG|TN|TZ|UG|YT|ZA|ZM|ZW
Lastly, use geojson-merge to merge the files and mapshaper to simplify the polygons and decrease the file size.