Since you are asking the question on HN, I assume you are working in an IT area. For that reason I suggest that EN-US should be basis for your "international" English. Simple reason being that the vast majority of resources are written EN-US, so why fight it. (I say that as an Aussie and a pedantic preference for EN-AU).
In order to overcome the differences in verbal communications, I suggest taking guidance from Amazon and focus on being documentation-centric. Ensure that a fluent English writer edits the documents. That way, non-native speakers can take their time to unravel materials and also improve their English.
I also find that it builds bridges to learn, to at least tourist level, the languages of your co-workers. Take time to learn about their customs, etc. It is also a good idea for persons who habitually speak fast to slow down. Give people more processing time to grok what is being said.
We speak international English together and it works well. Everyone does mistakes, everyone has a strong accent.
People from Texas or Scotland may be native speakers but they are not necessarily easier to understand than people from Italy or Vietnam.
Some communicate better than others, but they are not necessarily the native speakers.
I think you should accept that international English is not perfect British English and it’s okay for it to be a bit broken. Of course you need to clarify when confusion is possible. Eventually everyone will improve.
It's unavoidable, and a cost of doing business internationally.
IMO the personal conflicts you describe that arise in your situation aren't from the language challenges, it's from the company culture. Figure out how to extend those olive branches, be kind amd understanding to eachother, take the time to really connect and respect eachother and those problems will go away. In short, it's not a language problem but a culture/attitude problem. I've worked in teams like this that work well with minimal conflict, and ones that have lots of conflict. Both had the same language challenges, that's not the variable that's different between the two, its company culture and attitude.
So what can you do? Encourage your native speakers to simplify their language is a good first step. Stop using so many idioms that don’t directly translate, and pick more common words when more common words will do.
Then make sure that English is a skill assessed in hiring. And as it’s a business requirement for reasonable English, make sure that training is available to those that want / need it.
It doesn’t have to be English of course, the same advice would apply the other way around if you pick a different language. But you’ll probably find long-term hiring easier if you stick to a lingua franca.
I experienced English as lingua franca in an US company based in Germany with around 25 nationalities and there was no problem at all. In fact, it was a pleasure (and a great way to get travel recommendations!).
If the language level is acceptable, people will be able to convey their arguments appropriately. A native English speaker may be more fluent, but the argument can stand by itself, if the culture of the company is right. IMO if people cannot understand your argument at all, you do not have enough proficiency to work in that language.
One is language - do choose English as the lingua franca, and make sure the people you hire speak it well enough. To some extent, you need to hire people you can work with, and it starts with language. If you choose the local language, you just won't be able to hire.
The other one is much harder, and is culture. Basically, what might seem perfectly fine to a Dutch or a French, will sound exceedingly brutal or arrogant to a Brit, and this will lead to a neverending stream of misunderstandings, unless everyone's aware of it.
The same phenomenon will happen in non-technical positions and lead to overweight of English native speakers in management. Without native-level fluency, you are simply disadvantaged.
Meetings in a foreign language drain your energy very quickly, writing gives you the room to think.
This 50% split is pretty extreme, I don't know how that happened, you'd have to be in a field really lacking local talent or purposefully prioritize foreign workers over local ones. At every company I've worked at (all over Europe) we had 10 to 30% of workers from other countries and everybody naturally defaulted to the local language. It's hard, and often you have to switch to accommodate them, but it's always temporary. I work with an Indian guy, we both started around the same time a few years back, and back then I had to switch to English often, he'd stumble over his words and take a lot of time to express ideas, now I just had a zoom meeting with him: completely fluent in the local language, barely an accent.
1-2 years is not a long time (and you don't need that long to learn with full immersion for 8 hours a day), part of having employees is investing in them. The way I see it, it's not just for your own company's sake, it's your company's civic duty as it will allow them to continue working in this country and make it easier for their next employer. If your company has a training budget I'd set-up one on one language training with private tutors once a week. That's standard at my company (and subsidized by the state).
Your documentation should be in the local language as well. Your hiring practice should prioritize people who speak the language (wherever they come from) at a minimum level and keep those who don't at a reasonable percentage. Unless it's a full-remote international company and workers don't intend to move to your country.