I just had a thought, correct me if I'm wrong. IIRC, the Germans can't put any new guns on their capital ships. The smallest British caliber at the moment is 13.5" or 34.3cm. German calibers at the moment are 12"/30.5cm and 13.8"/35cm, which is larger than the British 34.3cm gun, making it illegal. As long as the Germans aren't allowed to mount a new gun on their ships for the duration of the treaty, that means they are effectively limited to 12".
I think the first is actually incorrect, while the second is correct. The WNT signatories are barred from developing new heavy naval guns, so the most powerful guns are Furious' 18"/45 guns followed by the South Dakotas' 16"/50 guns. Germany isn't a WNT signatory, so it can develop new heavy guns. It is under the Stockholm limits so it can't deploy new heavy guns for land service (not coastal defense), but has no restrictions on developing new heavy guns for naval use.
Trevayne has the right interpretation - the Germans cannot lay down new capital ships until 1928, and even then they can only be equipped with the smallest heavy gun in the RN (currently 13.5").
The Germans are free to develop new guns if they wish. The obvious thing for them to do at present would be to line down the 35-cm gun to 13.5"
Of course, as Germany emerges from its naval hibernation, the pressure to close these loopholes will grow.