Complete the Bengali (bn_BD) translation #470

The bn_BD catalog hasn't been updated since 2023. The filesize labels (%d Byte / %d Bytes) added in #294 are missing, and nonillion, decillion and googol are untranslated. million is marked fuzzy, so it gets skipped at runtime and intword(2_000_000) returns "2.0 million" in English. 1 year, %d day is also mistranslated, so naturaldelta(timedelta(days=370)) returns "5 বছর" (5 years).

I'm a native Bengali speaker and would like to send a PR fixing these.