GTA VI Language Support
The short answer
GTA VI supports 13 languages, including English, French, Italian, German, Russian, Japanese, Polish, Korean, Simplified Chinese, Traditional Chinese, Mexican Spanish, European Spanish, and Brazilian Portuguese.
What Rockstar/Take-Two said
The official Rockstar store list confirms these 13 languages and notes that only English has full dubbing.
What others said
No other source has changed this information.
What is still unconfirmed
There are no confirmed updates or additions to the language list.