Microsoft Teams change default language

If you want to change the default language of Microsoft Teams application, you can do this by following the below steps,

1. Open Teams
2. Click on your User icon (top right)
3. Select Settings
4. Scroll down a bit and you would see Language -> Restart to apply language settings -> App language
5. Choose the language you want to set using dropdown.

As of Sept 2020 the languages supported we can see in the dropdown are: Bahasa Indonesia (Indonesia), Català (Català), Cymraeg (Y Deyrnas Unedig), Dansk (Danmark), Deutsch (Deutschland), Deutsch (Liechtenstein), Deutsch (Luxemburg), Deutsch (Schweiz), Deutsch (Österreich), Eesti (Eesti), English (Australia), English (Belize), English (Canada), English (Caribbean), English (India), English (Ireland), English (Jamaica), English (Malaysia), English (New Zealand), English (Philippines), English (Singapore), English (South Africa), English (Trinidad y Tobago), English (United Kingdom), English (United States), English (Zimbabwe), Español (Argentina), Español (Bolivia), Español (Chile), Español (Colombia), Español (Costa Rica), Español (Ecuador), Español (El Salvador), Español (España, alfabetización internacional), Español (Estados Unidos), Español (Guatemala), Español (Honduras), Español (México), Español (Nicaragua), Español (Panamá), Español (Paraguay), Español (Perú), Español (Puerto Rico), Español (Republica Bolivariana de Venezuela), Español (República Dominicana), Español (Uruguay), Filipino (Pilipinas), Français (Belgique), Français (Canada), Français (France), Français (Luxembourg), Français (Principauté de Monaco), Français (Suisse), Hrvatski (Hrvatska), Italiano (Italia), Italiano (Svizzera), Latviešu (Latvija), Lietuvių (Lietuva), Magyar (Magyarország), Nederlands (België), Nederlands (Nederland), Norsk, Bokmål (Norge), Norsk, Nynorsk (Norge), Polski (Polska), Português (Brasil), Português (Portugal), Română (România), Slovenčina (Slovenská republika), Slovenščina (Slovenija), Srpski (Srbija), Suomi (Suomi), Svenska (Finland), Svenska (Sverige), Tiếng Việt (Việt Nam), Türkçe (Türkiye), Íslenska (Ísland), Čeština (Česká republika), Ελληνικά (Ελλάδας), Български (България), Русский (Россия), Українська (Україна), עברית (ישראל), العربية (المملكة العربية السعودية), हिन्दी (भारत), ไทย (ไทย), ქართული (საქართველო), 中文(中国), 中文(台灣), 中文(新加坡), 中文(澳門特別行政區), 中文(香港特別行政區), 日本語 (日本), 한국어 (대한민국)

Know More: https://support.microsoft.com/en-us/office/languages-supported-in-microsoft-teams-for-education-293792c3-352e-4b24-9fc2-4c28b5de2db8