Third party language implies that either citizens of country 1 must learn this third party language to the proficiency needed to understand legalese; or to both be expected to know, understand and apply the law but not be able to read it.
In addition, I can present the case of France, where judges must disregard laws that contradict treaties (the hierarchy of norms being Constitution > Treaties > Laws > Executive orders). Therefore treaties, even with no national law to implement them, have a direct effects on the law.
Once a treaty enters into effect, a nation is bound by the terms of the treaty even if they do not have implementing laws. However, in many cases treaties don't have any consequences if signatories don't pass implementing laws (see, e.g, the Kyoto Protocol).
Lawyer here — very few of us know more Latin than the odd artifact such as ejusdem generis or contra proferentem, which occasionally are relevant to contract interpretation. And those of us of a certain age who grew up Catholic will remember bits and pieces of the old Latin Mass. Otherwise, Latin is probably as dead to lawyers as to anyone else.
English is now the global lingua franca; while I used to be a grammar vigilante, not long ago it occurred to me that in the linguistic "marketplace," if we native speakers want English to outcompete, e.g., Mandarin or Spanish, we need to be willing to let the language evolve so that it's easier for non-native speakers to learn and use confidently.