I found a hacky way to do this:
First, I downloaded 19-Nimbus.bdf from
the rockbox source code. This contains only the English characters. It's in size 19.
Then, I downloaded the 16-GNU-Unifont.bdf. This covers most of the characters from all major languages. It's in size 16.
I opened both files in a notepad, and manually copy the "header" part and all the CHARSET that are in 19-Nimbus into 16-GNU-Unifont. The files are not difficult to understand! Use ENCODING to tell which character is which. And remember to include ENDFONT at the very end.
Finally, use convertbdf to convert the combined file to fnt. (I used the
Windows binary)
Again, this method is obviously not perfect as it combine two sizes with no regards to the spacing. For now, it suits my needs. If anyone can find a better way to do this. Please do let me know! Thank you.
19-Nimbus-16-Unifont.bdf19-Nimbus-16-Unifont.fnt