When 2 different entities that speak different languages have a treaty, there are 2 versions of the treaty, one for each language. Has there ever been a case where one of the entities had their version of the treaty be substantially different from the "official" version by the other country, without knowing that the official version was different and significantly to their detriment? I'm talking both about accidental and deliberate translations.
The Treaty of Waitangi (1840) between the British and Maori chiefs of North Island, New Zealand
The most critical difference between the texts revolves around the interpretation of three Māori words: kāwanatanga (governorship), which is ceded to the Queen in the first article; rangatiratanga (chieftainship) not mana (leadership) (which was stated in the Declaration of Independence just five years before the Treaty was signed), which is retained by the chiefs in the second; and taonga (property or valued possessions), which the chiefs are guaranteed ownership and control of, also in the second article. Few Māori involved with The Treaty negotiations understood the concepts of sovereignty or "governorship", as they were used by 19th-century Europeans, and lawyer Moana Jackson has stated that "ceding mana or sovereignty in a treaty was legally and culturally incomprehensible in Māori terms".
More on this treaty can be found at the New Zealand History site where there is this similar, but perhaps clearer, summary:
The Treaty in Māori was deemed to convey the meaning of the English version, but there are important differences. Most significantly, the word ‘sovereignty’ was translated as ‘kawanatanga’ (governance). Some Māori believed they were giving up government over their lands but retaining the right to manage their own affairs. The English version guaranteed ‘undisturbed possession’ of all their ‘properties’, but the Māori version guaranteed ‘tino rangatiratanga’ (full authority) over ‘taonga’ (treasures, which may be intangible). Māori understanding was at odds with the understanding of those negotiating the Treaty for the Crown, and as Māori society valued the spoken word, explanations given at the time were probably as important as the wording of the document.
For some background,
Lieutenant-Governor William Hobson had the task of securing British sovereignty over New Zealand. He relied on the advice and support of, among others, James Busby, the British Resident in New Zealand. The Treaty was prepared in just a few days. Missionary Henry Williams and his son Edward translated the English draft into Māori overnight on 4 February. About 500 Māori debated the document for a day and a night before it was signed on 6 February.
Hobson and others stressed the Treaty’s benefits while playing down the effects of British sovereignty on rangatiratanga (chiefly authority). Reassured that their status would be strengthened, many chiefs supported the agreement.
Many examples throughout history but of the top of my head I can think of-
The Treaty of Wuchale between Italy and Ethiopia, where in the Italian version Ethiopia became a vassal or protectorate, but in the Ethiopian version they were more allies with the rights and privileges of a sovereign independent nation like foreign relations. The supposed breaking of which caused the First Italo-Abyssinian War.
Disputes over Article 17 regarding the conduct of foreign affairs led to the First Italo–Ethiopian War. The Italian version stated that Ethiopia was obliged to conduct all foreign affairs through Italian authorities, in effect making Ethiopia an Italian protectorate, while the Amharic version gave Ethiopia considerable autonomy, with the option of communicating with third powers through the Italians.1 The misunderstanding, according to the Italians, was due to the mistranslation of a verb, which formed a permissive clause in Amharic and a mandatory one in Italian. Wikipedia: Treaty of Wuchale
There were also many cases when the Japanese invaded Korea during the 16th century (I think) where all the messengers in order to not offend the host and thus suffer bodily or business damage in the respective country edited the messages to be more polite or softer often completely negating the original meaning.
However, keep in mind I got these two examples from various places including the internet (I believe the Armchair historian and Extra Credits though it has been a while) but I did skim read in order to double check my memory.
UN Resolution 242 demanding withdrawal from the occupied territories in Palestine after the 1967 war. Wikipedia entry
The English text has a different emphasis on territories, some might fit, whereas the matching French text seems to imply all. Pay particular attention to Article 1, subpart i. Specifically, des territoires in French, would be better translated as the territories. No idea which text was closer in spirit to the intent during negotiations or whether any intentional duplicity was meant by any party. It's really a very, very, slight difference.
So Israel claims some withdrawal achieves compliance with 242, but Palestine claims total withdrawal is needed to do that. There's plenty of other stuff going on, of course, so I wouldn't overemphasize this aspect, but differences in the 2 translations do play their part.
An important factor was the cultural and language differences between the First Nations and Europeans this led to misinterpretations between the two groups. While the government representatives “were the products of a literate society the First Nations came from an oral culture (Miller 139).” And language barriers “meant that First Nations didn’t fully understand the terms of the agreements. Both sides had translators, but it was impossible to translate the ideas behind decades of tradition…they didn’t realize they were giving up anything (Roberts 89).”
Another angle would be that a treaty text is in just one language, but held ambiguous, on purpose, and the explanation of content presented to one party made unambiguous, and misleading on purpose, only for the deceiving party later to enforce pacta sunt servanda on one interpretation, claiming that there never was any ambiguity.
That may happen even today in many commercial situations, but a very prominent case in history was Lüderitzland:
Lüderitzland, today part of the Sperrgebiet, was far bigger than Frederiks had thought. The contract specified its width as "twintig geograph'sche mylen" (20 geographical miles), a term that the tribal chief was not familiar with; 1 German geographical mile equals 4 arcminutes (7.4 kilometers), whereas the common mile in the territory was the English mile, 1.6 kilometers. Both Lüderitz and the signing witness, Rhenish missionary Johannes Bam, knew that Chief Frederiks had no understanding of geographical miles. He was only concerned about fertile land, and the shore of the Atlantic Ocean was of no value to his tribe. When Frederiks finally became aware that the land he sold comprised almost his entire tribal area, he complained to the German Imperial Government, but Consul-General Gustav Nachtigal died on his return voyage, and the complaint was never delivered. The dodgy contract became known as the "Mile Swindle", and Adolf Lüderitz was nicknamed "Lügenfritz" (lie buddy) by his fellow countrymen. In 1887 "even the Colonial Department of the Foreign Office doubted the validity of the treaty".
And that this was completely intentional deception is evidenced by Lügenfritz writing:
In December 1882, Lüderitz sent Vogelsang to Cape Town in South Africa to explore settlement possibilities in the southwest. He was advised there by the son of the missionary Carl Hugo Hahn, who worked in South West Africa, who pointed him to the bay of Angra Pequena as a favourable landing site. Vogelsang also learned that mineral resources, e.g. copper, were to be expected in South West Africa.
After Vogelsang had the first accommodations for his expedition built in the bay of Angra Pequena in April 1883, he concluded a contract with the Nama captain Josef Frederiks II on May 1 in which the bay of Angra Pequena and the land within a radius of five geographical miles were sold to the Lüderitz company for 100 pounds in gold and 200 rifles. Vogelsang left it open whether it should be German miles to approx. 7.5 km or the shorter English miles to approx. 1.6 km. Since Lüderitz later proceeded from the German unit of measurement, the Nama saw themselves deceived, however, despite violent protests could not enforce their point of view. In August of the same year a second contract was concluded in which Lüderitz was sold the coastal strip between the Orange River and the 26th parallel and an area of 20 miles inland from each point of the coast for another 500 pounds and 60 rifles. Lüderitz wrote to his agent Vogelsang: "But let Joseph Fredericks believe for the time being that this is about 20 English miles." The questionable contractual bases of the acquisitions, commonly also called "mile fraud", brought Lüderitz the derisive name Lügenfritz early on.
Thereupon Lüderitz turned to the German Foreign Office with the request for protection for his possessions. As Imperial Chancellor Otto von Bismarck was sceptical about German colonial efforts, Lüderitz only received the vague promise that he would be granted the usual protection as any German abroad.
Evidently this was just a dirty trick, and looking closely it had neither validity – at least not in the scope intended – nor any real influence on subsequent events, other than being a legal fiction offering a convenient excuse to take and to hold by force what they wanted.
On July 3, 1754, British Colonel George Washington surrendered the improvised Fort Necessity to the French. The articles of capitulation that he signed were in French, and included an admission of guilt for “L'assasin” and “l’assasinat” (the assassination) of one of their officers.
Washington later claimed that this word had been translated to him as “the death” or “the loss” of an officer. This was especially embarrassing to him because an English gentleman at that time was supposed to know French.
There's a European union document called 'Study on Language and Translation in International Law and EU law' which has some examples of treaties with differences because of translations. It mentions the Italy and Abyssinia one already in one answer here and also the UN resolution 242 in another answer here but there are also some other examples.
A similar „translation error” occurred in a treaty signed in 1878 by the Kingdom of Spain and the Sultan of Sulu (Southern Philippines). According to the Spanish language version, Spain had complete sovereignty over the Sulu archipelago, while the Sulu official language version (Tausug) described its status as a protectorate. Interestingly, after the Philippine-American War, the peace treaty signed by the US and the Philippines based on the earlier Spanish treaty repeated the translation error: the English version referred to a complete dependency while the Tausug version described a protectorate.
Then it has an example of Article 10 of the Hague Convention on Service
An example of typical mistranslation is the English translation of the Spanish language declaration by Mexico to Article 10 of the Hague Convention on Service, regulating the service of judicial and extrajudicial documents which concerned the rejection by Mexico of alternative methods of service. According to the English translation, the rejection applied only to the service of documents by diplomatic or consular agents while the original Spanish version wished to cover all kinds of services. As a consequence, many US courts were misled by the English version of the Declaration and used registered mail for service purposes which, according to the Spanish version, could not be used as it was considered as an alternative method of service.
Also, the Hellgate Treaty of 1855 between the US government and Native American tribes located in western Montana. It's only in English I think but it was translated orally. Wikipedia says
A Jesuit observer, Father Adrian Hoecken, said that the translations were so poor that "not a tenth of what was said was understood by either side"
The end result of this was that the Indians thought they could keep their land forever but
Based on the terms of the accord, the Native Americans were to relinquish their territories to the United States government in exchange for payment installments that totaled $120,000 dollars.