Any one has a trouble with Vietnamese message (unicode trouble) on Instinct Solar? How can we install Vietnamese for this?

You'd need the APAC (Asia Pacific) version of Instinct for the support of Vietnamese and other Asian languages and alphabets. The standard ROW (Rest of World) model supports only the following languages / alphabets:

Languages Selectable on Garmin Instinct Watches | Garmin Customer Support 

• Български (Bulgaria)

• Čeština (Czech)

• Dansk (Danish)

• Deutsch (German)

• Eesti (Estonian)

• Eλληνικά (Greek)

• English

• Español (Spanish)

• Français (French)

• Hrvatski (Croatian)

• Italiano (Italian)

• Latviešu (Latvian)

• Lietuvių (Lithuanian)

• Magyar (Hungarian)

• Nederlands (Netherlands)

• Norsk (Norwegian)

• Polski (Polish)

• Português (Brazilian)

• Русский (Russian - East Slavic)

• Română (Romanian)

• Slovenčina (West Slavic)

• Slovenščina (South Slavic)

• Suomi (Finnish)

• Svenska (Swedish)

• Türkçe (Turkish)

• Yĸpaïaϲbĸa (Ukrainian)

Thank you. I already have one with NOT APAC I think? How can I install the new firmware with APAC?