You can't. Bengali is not one of the languages supported in the UI, so the device doesn't load the font you need automatically. You have three options:
- Continue to use lensmann's approach to force the device to load the needed glyphs.
- Edit the metadata of every book to replace the glyphs with ones the device can display normally, i.e. transliterate all the book and author titles into English characters.
- File a feature request on the Kobo website and hope the developers add Bengali as an officially supported UI language some day. Continue to use #1 or #2 until then.