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Treehouse is the product development division of Nintendo of America. Though the name is often used to refer solely to the localization department, "Treehouse" englobes the localization team, audio-visual department, product management and quality assurance work.
Treehouse handles English, French, and Spanish localizations for the North American market and at one point planned to expand to Brazilian Portuguese. Treehouse started as an initiative to address the lackluster quality of the English localization of Nintendo games during the 8-bits and 16-bits era. The department often translates games as they are being developed and thus often communicates with the Japanese development teams . The Japanese developers also consult Treehouse to prevent overly Japanese-specific cultural content from being included in the games, as to ensure Nintendo games have an "international" feel.
The "Treehouse" name finds its root in Donkey Kong Country; the team handling the North American localization of the game was "locked away" from the rest of NOA due to Nintendo's high secrecy toward the game and was codenamed "Treehouse" after Donkey Kong's residence. As Treehouse expanded, the name stuck.
From its founding up to the early Wii years, Treehouse's English translations were used for the English script in all regions, with minor editing to account for difference in spellings between American and British English as well as replacing any potentially offensive content (examples of the later include removing an instance of the word "shag" in Super Paper Mario as well as the sentence "! I don't get my jolies doing nice thing" in WarioWare: Smooth Moves being changed to "I don't get my kicks doing nice things"). After Mario Party 8 was was recalled in the UK due to featuring the word "spastic" (a term considering benign in American but considered highly offensive in the United Kingdoms), American and European English scripts started to differ more, to varying degrees, ranging from games with largely similar translations but different object and characters names to completely different scripts. However, this practice seems to have been abandoned beginning with the Switch.
For a time, Treehouse's scripts were also used as the basis for translation in other languages. One example of this practice is the French translation of Paper Mario, which features many character names and dialogue clearly based on Treehouse's writing rather than the original Japanese script. This practice largely stopped after the release of the Nintendo GameCube, although Treehouse's own French and Spanish translations often share terminology with the English one.