During an online conference from Sony Computer Entertainment Korea, Hideo Kojima confirmed in a pre-recorded video that Metal Gear Solid V: The Phantom Pain will support Korean language. This means subtitles and menus will both be displayed in Korean.
At Taipei Game Show Kojima also confirmed traditional Chinese support, and at Igromir the developer announced improvements to the Russian language options, which were later added to Ground Zeroes with a patch, so it seems like the studio is serious about localization for this installment of the series.
Metal Gear Solid V: The Phantom Pain is set for a worldwide release later this year.
Source: SCEK, via @B1GN05E