I was able to get the English to Vietnamese dictionary working on my Kobo Clara Colour in a way you would expect as if it came from Kobo - meaning the dictionary text size is normal and not missing characters. The other dictionaries in this thread have a very small text size.
I got the dictionary from the google drive link in this thread:
https://www.mobileread.com/forums/sh...d.php?t=346747
If you just use that file you'll have complaints like others in this thread, because the built in Kobo fonts are missing characters for Vietnamese. To get the fonts to work, I needed to modify the Rakuten Serif and Kakugo UD fonts via firmware update. The method is outlined in the same post link above.
I attached the fixed fonts I needed for my Kobo to get this dictionary working. However to explain how to do it for yourself in case your device has a different default font...
For Rakuten Serif, I simply renamed the IBM Plex Serif static files to replace the files in the kobo firmware update. For Kakugo UD I used the Noto Sans variable font file (NotoSans-VariableFont_wdth,wght.ttf), renamed it, and modified the metadata as per the instructions in the comment section of this reddit post:
https://www.reddit.com/r/kobo/commen..._clara_colour/
Both fonts were downloaded from google fonts.
Long story short, it was trial and error to identify it was only these two fonts. One is used for the headings, and the other is for the rest. In my troubleshooting I also updated Rakuten Serif but it wasn't needed to fix the dictionary.
The firmware update is super easy. Maybe a bit scary sounding, but very easy. Worst case scenario you just install the original one the same way and you're back to normal.