How to convert eBooks with diacritics?

I have books in foreign language (Slovak) which has diacritics (š,č,ť,ľ,ž).

I'm Having a problem with converting PDB format files to mobi (or any other format readable by a Kindle device) properly. I used online converters but they aren't able to convert it correctly, as I always receive a question mark "?" instead of the diacritics mentioned above.

Does anybody know how to fix this? Some good Ebook converter software or some sort of a trick?

Thank you.

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