For the following 29 languages, the voice clone feature is available:

ar-ae - Arabic
bg-bg - Bulgarian
cs-cz - Czech
da-dk - Danish
de-de - German
el-gr - Greek
en-us - English
es-es - Spanish
fi-fi - Finnish
fr-fr - French
hi-in - Hindi
hr-hr - Croatian
id-id - Indonesian
it-it - Italian
ja-jp - Japanese
ko-kr - Korean
ms-my - Malay
nl-nl - Dutch
pt-pt - Portuguese
pl-pl - Polish
ru-ru - Russian
sk-sk - Slovak
ta-in - Tamil
tl-ph - Filipino
tr-tr - Turkish
ro-ro - Romanian
sv-se - Swedish
uk-ua - Ukrainian
zh-cn - Chinese

The full list of languages:

af-za - Afrikaans (South Africa)
am-et - Amharic (Ethiopia)
ar-ae - Arabic (U.A.E.)
ar-bh - Arabic (Bahrain)
ar-dz - Arabic (Algeria)
ar-eg - Arabic (Egypt)
ar-iq - Arabic (Iraq)
ar-jo - Arabic (Jordan)
ar-kw - Arabic (Kuwait)
ar-lb - Arabic (Lebanon)
ar-ly - Arabic (Libya)
ar-ma - Arabic (Morocco)
ar-om - Arabic (Oman)
ar-qa - Arabic (Qatar)
ar-sa - Arabic (Saudi Arabia)
ar-sy - Arabic (Syria)
ar-tn - Arabic (Tunisia)
ar-ye - Arabic (Yemen)
az-az - Azerbaijan
bg-bg - Bulgarian (Bulgaria)
bn-bd - Bangla (Bangladesh)
bn-in - Bangla (India)
bs-ba - Bosnian
ca-es - Catalan (Spain)
cs-cz - Czech
cy-gb - Welsh
da-dk - Danish
de-at - German (Austria)
de-ch - German (Switzerland)
de-de - German (Germany)
el-gr - Greek (Greece)
en-au - English (Australia)
en-ca - English (Canada)
en-gb - English (United Kingdom)
en-hk - English (Hong Kong)
en-ie - English (Ireland)
en-in - English (India)
en-ke - English (Kenya)
en-ng - English (Nigeria)
en-nz - English (New Zealand)
en-ph - English (Republic of the Philippines)
en-sg - English (Singapore)
en-tz - English (Tanzania)
en-us - English (United States)
en-za - English (South Africa)
es-ar - Spanish (Argentina)
es-bo - Spanish (Bolivia)
es-cl - Spanish (Chile)
es-co - Spanish (Colombia)
es-cr - Spanish (Costa Rica)
es-cu - Spanish (Cuba)
es-do - Spanish (Dominican Republic)
es-ec - Spanish (Ecuador)
es-es - Spanish (Spain)
es-gq - Spanish (Equatorial Guinea)
es-gt - Spanish (Guatemala)
es-hn - Spanish (Honduras)
es-mx - Spanish (Mexico)
es-ni - Spanish (Nicaragua)
es-pa - Spanish (Panama)
es-pe - Spanish (Peru)
es-pr - Spanish (Puerto Rico)
es-py - Spanish (Paraguay)
es-sv - Spanish (El Salvador)
es-us - Spanish (USA)
es-uy - Spanish (Uruguay)
es-ve - Spanish (Venezuela)
et-ee - Estonian
fa-ir - Persian (Iran)
fi-fi - Finnish
fr-be - French (Belgium)
fr-ca - French (Canada)
fr-ch - French (Switzerland)
fr-fr - French (France)
ga-ie - Irish
gl-es - Spain (Galician)
gu-in - Gujarati (India)
he-il - Hebrew (Israel)
hi-in - Hindi (India)
hr-hr - Croatian
hu-hu - Hungarian
id-id - Indonesian
is-is - Icelandic
it-it - Italian
ja-jp - Japanese
jv-id - Javanese
ka-ge - Georgian
kk-kz - Kazakh
km-kh - Khmer (Cambodia)
kn-in - Kannada (India)
ko-kr - Korean (South Korea)
lo-la - Lao
lt-lt - Lithuanian
lv-lv - Latvian
mk-mk - Macedonian
ml-in - Malayalam
mn-mn - Mongolian
mr-in - Marathi
ms-my - Malay (Malaysia)
mt-mt - Maltese
my-mm - Burmese
nb-no - Norwegian
ne-np - Nepali
nl-be - Dutch (Belgium)
nl-nl - Dutch (Netherlands)
pa-in - Punjabi (India)
pl-pl - Polish
ps-af - Pashto (Afghanistan)
pt-br - Portuguese (Brazil)
pt-pt - Portuguese (Portugal)
ro-ro - Romanian
ru-ru - Russian
si-lk - Sinhala (Sri Lanka)
sk-sk - Slovak
sl-si - Slovenian
so-so - Somali
sq-al - Albanian
sr-rs - Serbian
su-id - Sundanese (Indonesia)
sv-se - Swedish (Sweden)
sw-ke - Swahili (Kenya)
sw-tz - Swahili (Tanzania)
ta-in - Tamil (India)
ta-lk - Tamil (Sri Lanka)
ta-my - Tamil (Malaysia)
ta-sg - Tamil (Singapore)
te-in - Telugu (India)
th-th - Thai (Thailand)
tl-ph - Tagalog (Philippines)
tr-tr - Turkish
uk-ua - Ukrainian
ur-in - Urdu (India)
ur-pk - Urdu (Pakistan)
uz-uz - Uzbek (Uzbekistan)
vi-vn - Vietnamese
zh-cn - Chinese (China mainland)
zh-cn-liaoning - Chinese (Liaoning)
zh-hk - Chinese (Hong Kong)
zh-tw - Chinese (Taiwan)
zu-za - Zulu (South Africa)