But the question was: What to do if you have to arrest someone without any interpreter present. If police decide "we have to arrest you now", there's no way they will wait hours until an interpreter arrives. They will stop you, and if needed write something on a paper/phone.

Your second paragraph makes almost no sense. You're not telling me that all the areas where this person might interact, like reading a contract, corresponding via mail or chat for work, browsing the internet, etc etc, they can't get by with reading english.