Are there confirmed cases where a country changed its language without being conquered?

I believe Alaska would also count. I can't imagine there were many (if any) English speakers living there before it was sold to the United States in 1867.

The Union of South Africa established in 1910 had English and Dutch as official languages. This was changed in 1925, where Afrikaans was added as a form of Dutch. In 1986, all mention of Dutch was scrapped, so it was Afrikaans only. In 1994, nine native languages were added.

100 years ago, France had many different local languages, such as Breton, a Celtic language. WP says that as late as 1950, there were about a million native Breton speakers. Today the language is for all practical purposes extinct.

One fairly typical scenario for this is the conquest of a foreign country, followeed by the assimilation of the new elite into the conquered population. A good example are the Manchu in China who assimilated so well that their language had becomme extinct at the end of the (Manchu) Qing dynasty. Other examples would be the Mongols in Iran, Russia and Central Asia, or the Bulgars in the Balkans

One can consider the opposite case: switching the language when the country gains its independence. The new langauge certainly exists, but not all inhabitants of the country may master it, and it may lack written forms or significant representation in the culture and politics. One can consider former Soviet republics as examples, and probably Ireland.

Remark on Hebrew: It is incorrect to say that Hebrew is "an artificial language" (see Constructed language) and it was created from "none": Biblical Hebrew existed as a liturgical language, known to practicing Jews all over the world. The pronunciation was also relatively standard, since being codified by Masoretes. However it has undergone many innovations, such as:

• developing vocabulary adapted modern realities (basic words like pen, pencil, monthly, etc.)
• reconstructing some grammatical forms on the basis of of the parallel forms in the related languages (such as Arabic)
• some grammatical changes due to being used by the speakers of Indo-European languages: notably switching to subject-verb-object word order, and transformation of perfect/imperfect tenses to represent past/future tense and active participle to represent the present tense.

There are of course many language reforms being performed from time to time, as partially listed in other answers. I won't mention language change that happened after the country stopped being "conquered" and as a result of being "free", and focus only on majority languages (there are way too many minority languages being made official in some partial function). However, there are indeed some (recent) examples when there was really a change in language (as in "a language is a sociolinguistic construct, not a mere linguistic one"):

• Montenegro officially changed its language from Serbocroatian to Montenegrin on 22 October 2007, but the language remained "the same" until some standardization changes in 2009/2010, which made it slightly different from standard Serbian.
• Bosnia and Hercegovina (or better, Federation of Bosnia and Herzegovina) slowly (since the 1990s) introduced standard Bosnian language as one of the official languages
• borderline example is Moldova, where the language has been (since the independence from the USSR) called on and off Romanian and Moldovan, depending on then current political situation (without any changes to the language itself)
• Singapore's Chinese population changed its languages from Cantonese to Mandarin around the 1980s; this was quite voluntary, because Mandarin gave much better economic opportunities. (Before anyone comments, Cantonese and Mandarin are two different, mutually unitelligible languages)
• China switched its written official language from Wényánwén to Pǔtōnghuà (these languages are considerably different) slowly from the beginning of the 20th century until after the communist revolution. The situation with the official standard spoken language has been considerably more complicated, but by the end of 20th century, Pǔtōnghuà has firmly become the spoken standard.
• Malta replaced Italian with Maltese as one (the second being English) of the official languages in 1934 (it was however a part of the British Empire, but this still a nice example)
• Kingdom of Hungary was still using Latin as its official language until 1836, then switched to Hungarian (it can be argued that it has been "conquered" during all the time, but this did not influence its language choice)
• USSR adopted Russian as the official union language in 1990 (it was "only" a language of the Russian Federation, one of the union republics until then).

The Norman's adopted French as their language even though they were the ones who conquered.

All of the countries under Roman rule gradually migrated away from common Latin into their own distinctive languages. One example is French, which is a modification and combination of Gaulish and vulgar Latin. England reverted to English as the official tongue of the court sometime in the 13th century after 300 years of French.

Indonesian

When Indonesia got independant from The Netherlands after WWII, they adopted Indonesian, a variation of Malay, as the official language of their newfounded state.

In 1945, when Indonesia declared its independence, Indonesian was formally declared the national language, despite being the native language of only about 5% of the population.

This choice met several purposes:

• Getting rid of Dutch, the language of the colonisator.
• Unifying the country with a common language.
• Promoting equality of citizens, by adopting a language without class distinctions, hence not Javanese, the most widly spoken language in Indonesia at that time.

With thousands of islands and hundreds of different languages, the newly independent country of Indonesia had to find a national language that could realistically be spoken by the majority of the population and that would not divide the nation by favouring one ethnic group, namely the Javanese, over the others.

Indeed, already three decades beforehand, the Youth Pledge defined Indonesian as bahasa persatuan, the language of unity.

Nowadays, more than 200 millions Indonesians speak Indonesian, even if

Most Indonesians, aside from speaking the national language, are fluent in at least one of the more than 700 indigenous local languages.

Although there were many languages spoken throughout the Byzantine Empire (or Eastern Roman Empire), its court and administration language switched from Latin to Greek over time.

Ancient Aramaic was adopted as a "lingua franca" by most of the Mesopotamian states in the eleventh century. The Neo-Assyrian Empire, an Akkadian speaking culture, managed to conquer the Aramaean cities, a major center of trade, early on, which included internal exile for the Aramaean elite throughout the Assyrian empire. The meant the ruling class and clerical class Assyrians were likely to also speak Aramaic, making it easier for inter-city trade within and without the empire.

It's almost analogous to the Roman elite adopting Greek after the conquest of Greece, except commoners and nearby trade partners also adopted the conquered tongue as well, while the conquering Assyrians retained their own language as their primary tongue for a while. (And this would get weird when the Chaldeans took over, and energetically re-established Akkadian as the language of the empire.)

Turkey, under the leadership of Mustafa Kemal Atatürk, introduced a series of language reforms via the public education system that seriously impacted comprehension of older documents. The changes included massive vocabulary changes as well as a switch from the Arabic to the Roman alphabet. Since the taxonomy of languages is always up for debate, it can be reasonably argued that a change to the Turkish language at this magnitude constitutes a shift to a new language, even if that new language is also called Turkish.

This change differs from the more gradual changes that typified transitions between classical, medieval, and modern languages (e.g. Latin to French, Middle English to Modern English, Old Norse to Danish, etc.) because it was quick, deliberate, and official. This was not a slow social process of changes filtering out through towns and villages and across social classes over decades or centuries, it happened via government decrees and official public school curriculum updates.

In the Grand Duchy of Lithuania, Ruthenian gradually became the main official language replacing Lithuanian. After the country became a part of the Polish-Lithuanian Commonwealth, Ruthenian was superseded by Polish.

Many countries have changed their official languages.

South Africa had two official languages (Afrikaans and English) before 1994 and now has 11.

Bolivia used to have only one official language (Spanish), but now it has 37 (yes, 37).

Rwanda went from having French and Kinyarwanda as its official languages to having English and Kinyarwanda, and now has English, French and Kinyarwanda.

In the U.S., Louisiana went from being French-speaking when it gained statehood to becoming bilingual and then in the 1920s actually banned French altogether, before trying to restore it in recent years.

You can find lots of other examples if you look around.

Of course, it happened many times when the country was conquered, later got its freedom and returned to the previous language, that it used before that last conquest. Sometimes the language even started to change before the real freedom came, as in Czech lands in 19th century, when they started to learn they own almost dead language.

The question is formulated obviously by a person, who lives in the mono-language culture. There are others, too. In the older Europe people very often used many languages - one at home, other in the village, third in the town, two more - with traders. For example, in Dagestan even now there are villages, where 2 languages are used that are no used in any other place. Of course, they also know 2-3 wider known Dagestan languages and Russian, too. How do you count these people?

And the picture in all old Europe was as that one. There were very many languages in France, in Germany, really in any country. They were no dialects, they were really very different.

How about the Holy Roman Empire?

Joseph II (Holy Roman Emperor, King of Germany etc) changed the language of the Empire to German in 1781 (Patent of Toleration), from Latin, and sometimes from local languages.

Here is the thing: there is some inherent, anachronistic assumption in the question about the language of a country. Before the XXth century, without public education, without media or even general literacy people spoke whatever they wanted. The language of a country was formalized only in things like the language of administrative offices, military command and such. 99% of the population didn't need to speak these at more than an elementary level (if at all).

The Roman Empire, at it start under emperors Julius Caesar (44 BC) and Augustus (27 BC) had Latin as its main language, and the one spoken by its elites and leaders. At the end of it, with the capture of Constantinople by the Ottomans in 1453, the prevailing language of the elites was Greek. The main change came under emperor Heraclius (610 to 641), whose reforms upon taking the throne included the introduction of Greek as the official language.