add Vietnamese translation

[NTP-996]

No description provided.

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[GianfrancoBazzani]

Hello @NTP-996, thanks for the contribution! I noticed the currently the PR mixes English and Vietnamese (only instances.md and instances_complete.md are translated). Could you translate the remaining level descriptions into Vietnamese as well? I will merge the PR after

[NTP-996]

Hi @GianfrancoBazzani, I just updated the translation