So I've released an update (V0.34.4, that has Vietnamese language support for directions (looks like it was actually already here, but a bug in paged item menu caused that it was not visible in the menu ) and directions filter improvements from Geoff Kuenning (who already helped to improve spoken directions last summer). Thanks Geoff !

How to enable spoken direction in Vietnamese (or any other supported language) ?

Select the language in Options->Navigation->Language->Language for directions.

Please not that the distance announcements are not (yet) translated and are in English for all languages.