There is no single answer to this question, as each company has different needs and objectives. That said, in an ideal scenario, each country or at least each language should have its own separate account within Google Ads. Separate accounts make multi-language and multi-country campaign management more effective because everything is well organized and separate: pixels, conversions, audiences, losses, extensions..., allowing different specialists to work together in an organized manner.
Likewise, ideally each client should have all accounts integrated within a MCC of their own. This way, the client can connect or disconnect their MCC containing all their accounts at will, granting or denying access to any agency as needed.