The Wayback Machine - https://web.archive.org/web/20151028170623/https://dev.twitter.com/web/overview/languages

Twitter for Websites supported languages

Translated widget text is available in 34 of the languages supported by Twitter.com.

Languages supported by Twitter for Websites widgets and buttons
NameLanguage code
English (default)en
Arabicar
Bengalibn
Czechcs
Danishda
Germande
Greekel
Spanishes
Persianfa
Finnishfi
Filipinofil
Frenchfr
Hebrewhe
Hindihi
Hungarianhu
Indonesianid
Italianit
Japaneseja
Koreanko
Malaymsa
Dutchnl
Norwegianno
Polishpl
Portuguesept
Romanianro
Russianru
Swedishsv
Thaith
Turkishtr
Ukrainianuk
Urduur
Vietnamesevi
Chinese (Simplified)zh-cn
Chinese (Traditional)zh-tw

A language code may be set automatically by Twitter’s markup generators based on a logged-in Twitter user’s language setting or a language chosen in the markup generator.

<a
  class="twitter-share-button"
  href="https://twitter.com/share"
  data-lang="es">
Twittear
</a>

Twitter’s widget-js library will extract the most appropriate language from your widget’s DOM context if no language is provided in the widget markup. Pages which define a script or region subtag not supported by Twitter widgets will be mapped to the closest available language: e.g. pt-BR will use pt if Portuguese is available but a Brazilian regional localization is not.

United States English is the default if a page’s language code does not match any available translation.

Note: setting the language only affects the language of Twitter elements such as action text and timestamp display; Tweet text is always displayed in its originally authored language.