There's just too much nuance lost. French has very particular words for concepts of state philosophy, politics, and law, and so has German (eg. yesterday's result saw the Green party's uprising, with their somewhat gender-mainstreamed and weaseled, but in any case special language), and other European languages as well I guess. English could be easily seen as the language of eg. finance-dominated Eurocracy, so using plain language also helps in not portraying EU institutions as "slaves to an international capitalist elite" or something, and give arguments to radical parties.
That's exactly why English is such a success. By losing nuance in the words but making up for it in the sentences the language becomes a lot easier to learn.
I'd much rather have the common language of the EU be English even without an English representation in the EU than to have to parse EU legal documents in French or German. The occasional contract is bad enough. At least with English there is a level playing field.
But a major practical concern is that after Brexit the EU doesn't have Common Law countries left (except maybe Ireland and Malta, but I don't know really). For law and contract texts, Code Civil (Napoleonic Law) will be even more prevalent than before. So I can't see a good reason to stick to English which is simply an awkward language for expressing continental civil law concepts when there's a very large body of law and court language use in the respective native language.
I really can't follow your argument. The EU laws are brand new and not usually connected to the civil code of the countries of the EU, who all have their own take on a lot of stuff. Case in point: the GDPR. The English text is used as the reference by the various countries that have now implemented it, there are translations but anything before the EU courts will most likely be conducted in English and use the English text. Regardless, those texts can be translated and nothing stops a country that is worried about its citizenry being unable to read the text to translate it.
I don't see any major influence of either common law or the Napoleonic civil code on the EU legislation, though the court process arguably has been influenced strongly by those principles.
> The English text is used as the reference by the various countries that have now implemented it,
Is it? All EU laws are published in the official languages of all members. Legally, there are no translations but 22 texts that have equal status and the same meaning.
Don't ask me what happens if the meanings turn out not to match exactly in a relevant way.
Has there been a case before the EU high court yet where there was a meaningful discrepancy between a local version and another that was deemed to be leading?