Hreflang is unusually unforgiving: a single malformed entry causes the entire cluster to be ignored, with no error anywhere.
The annotation count explodes
Each page must reference every version including itself, so n language versions produce n² annotations across the set. Ten locales over a thousand pages is a hundred thousand annotations. Generate them from data; do not template them by hand.
Codes are ISO, and the obvious guess is often wrong
Language is ISO 639-1, region is ISO 3166-1 alpha-2. The United Kingdom is gb, not uk. Japanese is ja, not jp. Chinese needs a script subtag — zh-Hans or zh-Hant — because simplified and traditional are not distinguished by region alone.
Absolute URLs and one canonical per version
Relative URLs are invalid here. Each annotated URL must also be self-canonical: if the German page canonicalises to the English one, you have told the engine both that they are alternatives and that one does not exist independently. That contradiction voids the cluster.
Always declare x-default
It catches every locale you did not enumerate. Without it, unmatched visitors get whichever version the engine guesses.
Try it: Hreflang Tag Generator on SeoWolf's Notepad.