The StackOverflow post you linked to is seven years old. Based on the Web Speech API demo at
[['Afrikaans', ['af-ZA']],
['አማርኛ', ['am-ET']], → Amharic
['Azərbaycanca', ['az-AZ']],
['বাংলা', ['bn-BD', 'বাংলাদেশ'], → Bengali, Bangladeshi
['bn-IN', 'ভারত']], → Bengali, Indian
['Bahasa Indonesia',['id-ID']], → Indonesian
['Bahasa Melayu', ['ms-MY']], → Malay
['Català', ['ca-ES']],
['Čeština', ['cs-CZ']], → Czech
['Dansk', ['da-DK']],
['Deutsch', ['de-DE']],
['English', ['en-AU', 'Australia'],
['en-CA', 'Canada'],
['en-IN', 'India'],
['en-KE', 'Kenya'],
['en-TZ', 'Tanzania'],
['en-GH', 'Ghana'],
['en-NZ', 'New Zealand'],
['en-NG', 'Nigeria'],
['en-ZA', 'South Africa'],
['en-PH', 'Philippines'],
['en-GB', 'United Kingdom'],
['en-US', 'United States']],
['Español', ['es-AR', 'Argentina'],
['es-BO', 'Bolivia'],
['es-CL', 'Chile'],
['es-CO', 'Colombia'],
['es-CR', 'Costa Rica'],
['es-EC', 'Ecuador'],
['es-SV', 'El Salvador'],
['es-ES', 'España'],
['es-US', 'Estados Unidos'],
['es-GT', 'Guatemala'],
['es-HN', 'Honduras'],
['es-MX', 'México'],
['es-NI', 'Nicaragua'],
['es-PA', 'Panamá'],
['es-PY', 'Paraguay'],
['es-PE', 'Perú'],
['es-PR', 'Puerto Rico'],
['es-DO', 'República Dominicana'],
['es-UY', 'Uruguay'],
['es-VE', 'Venezuela']],
['Euskara', ['eu-ES']], → Basque
['Filipino', ['fil-PH']],
['Français', ['fr-FR']],
['Basa Jawa', ['jv-ID']], → Javanese
['Galego', ['gl-ES']], → Galician
['ગુજરાતી', ['gu-IN']], → Gujarati
['Hrvatski', ['hr-HR']], → Croatian
['IsiZulu', ['zu-ZA']], → Zulu, South Africa
['Íslenska', ['is-IS']], → Icelandic
['Italiano', ['it-IT', 'Italia'],
['it-CH', 'Svizzera']],
['ಕನ್ನಡ', ['kn-IN']], → Kannada
['ភាសាខ្មែរ', ['km-KH']], → Khmer
['Latviešu', ['lv-LV']], → Latvian
['Lietuvių', ['lt-LT']], → Lithuanian
['മലയാളം', ['ml-IN']], → Malayam
['मराठी', ['mr-IN']], → Marathi
['Magyar', ['hu-HU']], → Hungarian
['ລາວ', ['lo-LA']], → Lao
['Nederlands', ['nl-NL']], → Dutch
['नेपाली भाषा', ['ne-NP']], → Nepali
['Norsk bokmål', ['nb-NO']],
['Polski', ['pl-PL']],
['Português', ['pt-BR', 'Brasil'],
['pt-PT', 'Portugal']],
['Română', ['ro-RO']],
['සිංහල', ['si-LK']], → Sinhala
['Slovenščina', ['sl-SI']], → Slovenian
['Basa Sunda', ['su-ID']], → Sundanese
['Slovenčina', ['sk-SK']],
['Suomi', ['fi-FI']], → Finnish
['Svenska', ['sv-SE']],
['Kiswahili', ['sw-TZ', 'Tanzania'],
['sw-KE', 'Kenya']],
['ქართული', ['ka-GE']], → Georgian
['Հայերեն', ['hy-AM']], → Armenian
['தமிழ்', ['ta-IN', 'இந்தியா'], → Tamil, from India
['ta-SG', 'சிங்கப்பூர்'], → Tamil, from Singapore
['ta-LK', 'இலங்கை'], → Tamil, from Sri Lanka
['ta-MY', 'மலேசியா']], → Tamil, from Malaysian
['తెలుగు', ['te-IN']], → Telugu
['Tiếng Việt', ['vi-VN']],
['Türkçe', ['tr-TR']],
['اُردُو', ['ur-PK', 'پاکستان'], → Urdu
['ur-IN', 'بھارت']],
['Ελληνικά', ['el-GR']], → Greek
['български', ['bg-BG']], → Bulgarian
['Pусский', ['ru-RU']], → Russian
['Српски', ['sr-RS']], → Serbian
['Українська', ['uk-UA']], → Ukrainian
['한국어', ['ko-KR']], → Korean
['中文', ['cmn-Hans-CN', '普通话 (中国大陆)'], → Mandarin, from China
['cmn-Hans-HK', '普通话 (香港)'], → Mandarin, from HK
['cmn-Hant-TW', '中文 (台灣)'], → Mandarin, from Taiwan
['yue-Hant-HK', '粵語 (香港)']], Cantonese, from HK
['日本語', ['ja-JP']], → Japanese
['हिन्दी', ['hi-IN']], → Hindi
['ภาษาไทย', ['th-TH']]]; → Thai
(Do please include Lao! The Lao STT is terrific!)
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