An international site with proper translations still serves the wrong language version in search results. The translations are fine; the annotations connecting them are not.
The symptom
German searchers see the English page ranking for German queries. The German page exists, is indexed, and is not being chosen — because nothing tells the search engine the two are alternatives rather than competitors.
The commercial cost
A visitor landing in the wrong language usually leaves. Every one of those is a lost sale from a customer who searched for exactly what you sell, and the analytics report it as a bounce with no obvious cause.
Language and country are not the same
es covers Spanish everywhere. es-mx targets Mexico specifically. If pricing, currency or shipping differ by country, you need regional codes; if only the language differs, country codes fragment your signals for no benefit.
Try it: Hreflang Tag Generator on SeoWolf's Notepad.