Bispell is a simple editor of plain text capable of spell checking a bilingual text. If you have a text written in two languages, and the two languages are not two big blocks, it's desirable to have a tool to detect in what language each piece of the text was written and spell check accordingly. Currently, as far as I know, the most capable free software text processors can spell check multilingual texts, but they can't guess in what language the text was written. Bispell is useful, for example, if your text is written in one language but with many quotations in another language.
These details are provided for information only. No information here is legal advice and should not be used as such.