The PocketBook devices have their own proprietary format for dictionaries. They provide a program for converting some types of dictionaries to their format. You can download it from the website (it's called something like DictionaryConverter). Doing the conversion properly is rather involved, so I won't go into it here. There are some threads here that discuss the details, and you should look at them. I'm not sure if the converter will work on a Mobi file directly. Perhaps you can deconstruct the Mobi file and pull out the dictionary from within it. Hopefully it will be in one of the standard formats (xdxf, stardict,...) that the converter knows about.
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