France is the classic case, in 1800, about 1/2 the population of France spoke another tongue. Brittany spoke Breton (a variation of Celtic), Southern France spoke Provençal, a Romantic language closer to Italian and Spanish then to French. Along the Rhine, German was spoken at home even when French had to be used in School. France still has such laws requiring French to be spoken,, through in recent decades it has permitted the German Speakers in Alsace-Lorraine to put up street signs in German, the language of the Majority of residents in that area of France to this day.
Now, Brittany and Provence have suffered rapid decline in the last 200 years do to the efforts of the French Government to have everyone in France Speak French. This was less effective in Alsace-Lorraine for it was part of Germany from 1870-1918 and again from 1940-1945.
Breton, spoken by 1.2 million people in 1930, only 200,000 today:
http://en.wikipedia.org/wiki/Breton_languageFranco-Provence, spoken in Switzerland and that area of France next to Switzerland:
http://en.wikipedia.org/wiki/Franco-Proven%C3%A7al_languageWhat in English is referred to a Provençal Language but more accurate name is Occitan language:
http://en.wikipedia.org/wiki/OccitanOnce the primary language in Southern France in 1800, now spoken by less then 600,000 people and then most of them also speak French.
http://en.wikipedia.org/wiki/File:Langues_de_la_France.svghttp://en.wikipedia.org/wiki/Languages_of_FranceAlsatian Language, the local variation of German spoken in Alsace-Lorraine, but speakers are down to 458,000:
http://en.wikipedia.org/wiki/Alsatian_languageNow, I picked France for we have a clearer record of French suppression of other languages then we do in other countries, but Britain had problems with non-English speakers. During the Reign of Queen Elizabeth I, when she ordered Mass to be said in English instead of Latin, Cromwell revolted. The reason was simple, they were use to Latin being used in Church and for official documents, but if the Language in Church had to be changed why did it have to be changed to a "Foreign" Language that was English to the people of Cromwell. Similar acts were later pass as to Wales, Scotland and Ireland for speaking anything but English. Please note all of this suppression dates to the 1500-1800 period, a period where France left its people speak whatever language they wanted to.
In recent years they have been a growing tendency to accept minority languages, but even today, when a treaty was proposed to protect such languages, the French Government ruled, that while it did sign the treaty, it could NOT ratify it for such ratification would elevate these minority languages to the same level of French, and under the French Constitution that is forbidden.
Thus the banning by Libya of the Barber Lang ague (The more generally accepted name for the Amazigh people) is not something new and shocking. Most countries have done the same over the last 200 or so years, especially western first world countries.
As to why, the reason seems to be control, if the Central Government can force everyone to speak their language, the central Government can control those people. Thus as the Nation-State developed and the Nationalize became the norm in Most countries, so did the desire to have only one language. Thus the French wanted everyone to Speak French, the English wanted everyone to Speak English, the Germans wanted everyone to Speak German, The Spanish wanted everyone to Speak Spanish. It seem Libya adopted the same rule, wanted everyone to speak Arabic so all education can be done by Arab speakers only. Given that permitted him to control who taught what, that gave Libya the ability to control who learned what. This is the same reason, England, France, Spain, Germany and most of the rest of the World adopted similar rules.