A Belgian auto parts supplier has forbidden its workers to speak any language other than Dutch

I wonder what happens if they speak Flemish, Frisian, or Afrikaans? All kidding aside, there are a bunch of Low Franconian dialects (or languages, depending on politics) to chose from. Frisian is technically outside the group, but shares some characteristics with Dutch. One wonders if a ban to speak proper Dutch could be enforced: no slang or words or usages not in the official dictionaries or grammar- / usage books. (Here's a lovely monolingual Dutch dictionary online.)

אַ שפּראַך איז אַ דיאַלעקט מיט אַן אַרמיי און פֿלאָט

A language is a dialect with an army and navy. [Max Weinreich] (Original saying was in Yiddish: a shprakh iz a dialekt mit an armey un flot.)

Ceci n'est pas un seing.