To the best of my knowledge, only Brazil and Saudi Arabia have a national motto (or whatever you want to call it) on their national flag. Many more have a motto on their official coat of arms, though and from there, it frequently gets onto passports, official documents and the currency.
So how does the motto get chosen? And what does that choice say about a country?
I suppose most mottos are chosen at independence, so the leaders of the nationalist or revolutionary movements are probably important. But can we learn about a country's history from a motto? Or, especially in Africa, the personality of its first President or 'founding father'?
Compare these two, for example:
The first is Côte d'Ivoire: serious, conservative, in keeping with President Houphoët's non-revolutionary ideology. The second is its neighbour Burkina Faso: superficially similar (and following France's lead by doing things in threes) but more progressive-sounding, more socialist maybe, with a revolutionary tinge. Coincidence?
I don't think so. When Burkina Faso was Upper Volta, its motto was actually "Unité-discipline-justice". When Thomas Sankara renamed it 'The Land of Upright People' in 1984, he changed the flag, national anthem and motto as well. Out with discipline, in with progress. A subtle change, but a symbolic one.
Or how about these two in East Africa? Kenya has 'Harambee' ('Together') while Tanzania has 'Uhuru na umoja' (Freedom and unity'). Almost identical, you might think. But 'Harambee' wasn't just a slogan for Jomo Kenyatta, it became a defining ideology for Kenya: nationalism, economic development and the uniquely Kenyan institution of the 'harambee meeting', community fundraising events in which local dignitaries donate to worthy causes and politicians (pardon my cynicism) return some of the cash they have looted to the people. Meanwhile, Tanzania - then as now a lopsided federation - stresses unity. The word 'Uhuru' is important too: Tanzania's president Julius Nyerere published two collections of speeches with 'Uhuru' in the title ('Freedom and Socialism' and 'Freedom and Development')
As to why Kenya has lions on its coat of arms and Tanzania has a man and a woman, I am afraid to speculate, but the symbolic contrast is striking.
Some slogans are unintentionally ironic. Liberia's fine emblem (left) fails to mention is that only 2% or so of its population where brought there by "the love of liberty". The rest of them were probably as bewildered by the beautiful ship with the white sails as I would be if a bunch of white Baptist Americans showed up in northern England and announced they had come to settle there.
How about the choice of language? Do you choose a colonial language, or do you pick a local one (and thereby risk offending minority ethnic groups?). Most countries in West Africa seem to go with English or French, whereas Swahili rules in East Africa, maybe because it's a regional rather than a local language so there's less chance of offending someone. The British and the Dutch royal families both have French mottos, but nobody seems to care.
Sometimes national mottos resonate in unintended ways. Mali's motto is a super-idealistic "Un peuple, un but, une foi" ("One people, one goal, one faith"). At first I thought it sounded like a song by U2. Then I remembered where I had come across 'one people' before. The motto of Hitler's Germany was "Ein Volk, ein Reich, ein Führer". Thankfully, a motto does not always a people make.