Because of this kind of annoyances, we usually leave US English as default language in the model and add UK English and Dutch.
Our app template has a config entity where we configure the default language for new accounts. When creating a new account, that value is used so admins don't need to choose the language for each account. This can also be used in the custom microflows that can be specified or changed in most SSO solutions.
Some customers have Dutch only models and run into these translation errors as well. Even platform supported modules like NativeMobileResources miss some translations which have to be put back in everytime. Let alone the many community modules that are often US English only. In our apps, these appear in English which usually is just fine for the admins.
I see this quite a bit as an English UK user. My usual solution is to just copy all the translations from English US to English UK using Language → Language Options. It won’t lock you out of upgrades to the modules, it just provides the missing translations needed for language aware modules.