Naval treaties tried to stop wartime seizure of export ships

Did naval treaties forbid a shipbuilding power from taking over foreign warships in its yards when war began?

Yes. Article 17 of the Washington Naval Treaty prohibited using undelivered warships being built for another power, although Britain had a history of compulsory purchase in wartime.

Destroyer procurement involved the legal risk of ordering ships abroad. Article 17 covered warships under construction in a belligerent's jurisdiction for another state and barred their wartime use by the builder. The clause is presented as an explicit attempt to prevent Britain from repeating its practice of taking over export orders when hostilities began. The rule mattered to navies such as Turkey, Greece, Poland, and Argentina that relied on foreign yards. Alliance expectations and treaty credibility could therefore affect whether an ordered destroyer would ever reach its purchaser.