If you have access to such dictionary in the right format, then yes, it is easy to use it as you describe.
And you don't have to specify the language of the book, you just have to select the dictionary you want to use, in the result panel, when doing a specific search.
Either you patch the e reader and your added custom dictionary will be available as extra_e1 (for example) or you have to 'disguise' it as one of the default languages (you will loose access to that language, so pick one you are not planning to use).
There are a few tutorials in this forum for both options.
If you don't have the dictionary, you have always the option to eventually convert from another format, assuming you have access to the right data.
There are some tools available in this forum to achieve that goal. They seem quite simple to use, but I have never use them myself...
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