I know this is an old post, but since this information is annoyingly hard to find I thought I'd post a list for anyone who might be looking. Please leave a note if you find any errors or omissions.
{
"Afrikaans": [
["South Africa", "af-ZA"]
],
"Arabic" : [
["Algeria","ar-DZ"],
["Bahrain","ar-BH"],
["Egypt","ar-EG"],
["Israel","ar-IL"],
["Iraq","ar-IQ"],
["Jordan","ar-JO"],
["Kuwait","ar-KW"],
["Lebanon","ar-LB"],
["Morocco","ar-MA"],
["Oman","ar-OM"],
["Palestinian Territory","ar-PS"],
["Qatar","ar-QA"],
["Saudi Arabia","ar-SA"],
["Tunisia","ar-TN"],
["UAE","ar-AE"]
],
"Basque": [
["Spain", "eu-ES"]
],
"Bulgarian": [
["Bulgaria", "bg-BG"]
],
"Catalan": [
["Spain", "ca-ES"]
],
"Chinese Mandarin": [
["China (Simp.)", "cmn-Hans-CN"],
["Hong Kong SAR (Trad.)", "cmn-Hans-HK"],
["Taiwan (Trad.)", "cmn-Hant-TW"]
],
"Chinese Cantonese": [
["Hong Kong", "yue-Hant-HK"]
],
"Croatian": [
["Croatia", "hr_HR"]
],
"Czech": [
["Czech Republic", "cs-CZ"]
],
"Danish": [
["Denmark", "da-DK"]
],
"English": [
["Australia", "en-AU"],
["Canada", "en-CA"],
["India", "en-IN"],
["Ireland", "en-IE"],
["New Zealand", "en-NZ"],
["Philippines", "en-PH"],
["South Africa", "en-ZA"],
["United Kingdom", "en-GB"],
["United States", "en-US"]
],
"Farsi": [
["Iran", "fa-IR"]
],
"French": [
["France", "fr-FR"]
],
"Filipino": [
["Philippines", "fil-PH"]
],
"Galician": [
["Spain", "gl-ES"]
],
"German": [
["Germany", "de-DE"]
],
"Greek": [
["Greece", "el-GR"]
],
"Finnish": [
["Finland", "fi-FI"]
],
"Hebrew" :[
["Israel", "he-IL"]
],
"Hindi": [
["India", "hi-IN"]
],
"Hungarian": [
["Hungary", "hu-HU"]
],
"Indonesian": [
["Indonesia", "id-ID"]
],
"Icelandic": [
["Iceland", "is-IS"]
],
"Italian": [
["Italy", "it-IT"],
["Switzerland", "it-CH"]
],
"Japanese": [
["Japan", "ja-JP"]
],
"Korean": [
["Korea", "ko-KR"]
],
"Lithuanian": [
["Lithuania", "lt-LT"]
],
"Malaysian": [
["Malaysia", "ms-MY"]
],
"Dutch": [
["Netherlands", "nl-NL"]
],
"Norwegian": [
["Norway", "nb-NO"]
],
"Polish": [
["Poland", "pl-PL"]
],
"Portuguese": [
["Brazil", "pt-BR"],
["Portugal", "pt-PT"]
],
"Romanian": [
["Romania", "ro-RO"]
],
"Russian": [
["Russia", "ru-RU"]
],
"Serbian": [
["Serbia", "sr-RS"]
],
"Slovak": [
["Slovakia", "sk-SK"]
],
"Slovenian": [
["Slovenia", "sl-SI"]
],
"Spanish": [
["Argentina", "es-AR"],
["Bolivia", "es-BO"],
["Chile", "es-CL"],
["Colombia", "es-CO"],
["Costa Rica", "es-CR"],
["Dominican Republic", "es-DO"],
["Ecuador", "es-EC"],
["El Salvador", "es-SV"],
["Guatemala", "es-GT"],
["Honduras", "es-HN"],
["México", "es-MX"],
["Nicaragua", "es-NI"],
["Panamá", "es-PA"],
["Paraguay", "es-PY"],
["Perú", "es-PE"],
["Puerto Rico", "es-PR"],
["Spain", "es-ES"],
["Uruguay", "es-UY"],
["United States", "es-US"],
["Venezuela", "es-VE"]
],
"Swedish": [
["Sweden", "sv-SE"]
],
"Thai": [
["Thailand", "th-TH"]
],
"Turkish": [
["Turkey", "tr-TR"]
],
"Ukrainian": [
["Ukraine", "uk-UA"]
],
"Vietnamese": [
["Viet Nam", "vi-VN"]
],
"Zulu": [
["South Africa", "zu-ZA"]
]
}
Edit: I also found this list, which is probably more current:
https://cloud.google.com/speech-to-text/docs/languages
Edit 2: Adding this list of sample voices as well: https://cloud.google.com/text-to-speech/docs/voices