The Apple Watch uses a version of OS X (watchOS) which has different language capabilities than Mac OS X or iOS. Localizations for the menus and dialogues (set using the iPhone Apple Watch app via the menu My Watch > General > Language) are English (U.S.), English (UK), English (Australia), English (India), Spanish (Mexico), Spanish (Spain), French (France), French (Canada), German, Italian, Japanese, Korean, Chinese (Simplified), Chinese (Traditional), Chinese (Traditional, Hong Kong), Portuguese (Portugal), Portuguese (Brazil), Danish, Dutch, Swedish, Russian, Thai, Turkish, Finnish, Indonesian, Norwegian, Polish, Arabic, Czech, Greek, Hebrew, Hungarian, Malay, and Vietnamese.
Languages available for dictation, Siri, and Smart Replies may be somewhat different.
This page provides instructions on how to adjust the dictation language of the Watch.
This article provides some info on how watchOS features have evolved.
(Note: I don't have a Watch and Apple's published info is only fragmentary. Comments and additional info from readers is welcome)