Translations are not just about the languages by itself, but culture as well, so it is normal differents translations to different countries.
Maybe you can look at what is the best choice for you reading each translation (and maybe help to translate as well if you feel comfortable).
nl-BE: https://translate.www.ads-software.com/locale/nl-be
nl-NL: https://translate.www.ads-software.com/locale/nl
Both are fully translated into 4.9.x
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