>>2103802>Not concerned about this as long as theres consistency in requirements - don't want to carry a million different documents that all say the same thing.
My experience is mostly, as noted, in Europe, but I have the Swiss government issued Covid-Zertifikat on my phone, which has worked everywhere I have been.
And although I have heard horror stories of two people with monolingual test results (only in German or French, not also in English as is more standard) being being refused boarding at Zürich airport for a couple of flights (one to Russia, one to somewhere else east of here), my test results (from Switzerland, France, Spain, Italy, and Portugal) have all been in local languages plus English, and accepted without issue at all the airports where I have presented them.
I think the most documentation anyone might need would be both a negative PCR and whatever the official vax cert is. No idea about passengers from places like the States that don’t have an official certification document, though (I have the little yellow WHO card tucked into my passport, too, but I don’t think anyone has ever looked at it.)