Find a "universal" font

[g123k]

          off-dart currently supports [185 languages](https://github.com/openfoodfacts/openfoodfacts-dart/blob/master/lib/src/utils/language_helper.dart).

Both Roboto and Djakarta support more languages, and Public Sans supports less languages than Roboto.
To have a clear idea we need to check languages one by one, in order to filter to languages we do need.

Originally posted by @monsieurtanuki in #4361 (comment)

[monsieurtanuki]

@g123k For the record we cannot really trust https://fonts.google.com/specimen/Roboto/glyphs about which languages they support.
I've managed to type in something in Japanese in Smoothie (Roboto), while Japanese is not supposed to be supported.
And I've scrolled in the "select your language" dialog in Smoothie, and all languages were written without obvious "I don't know" characters.

[g123k]

We now use OpenSans
9a1ba90