Hi lilryu,
I think the Yamaha PSR-S970/S770 can only display western languages such as English (Yamaha calls it International), and Japanese on the Lyrics/Text screen. I tried playing MIDI songs with non-English lyrics / text, such as Chinese, and the PSR-S970 simply displayed garbage.
P.13 of Yamaha PSR-S970/S770 owner's manual:
When inputting lyrics in the Song Creator function (see the Reference Manual, Chapter 3), you may also enter Japanese characters (kana and kanji).
P.65 of the same manual:
When the lyrics are garbled or unreadable, you may need to change the Lyrics Language setting:
[FUNCTION] -> TAB [E] MENU 1 -> [H] SONG SETTING -> Others
Only 3 choices for lyrics language: Auto, International, Japanese:
If Auto and International lyrics language options still cannot resolve your Vietnamese character display, then Vietnamese characters are not supported by these keyboards.
Thanks,
Paul