"I read somewhere once (i.e. I can't remember where!) that there was a point at which German was nearly adopted as the national language!"

Seems unlikely. Perhaps you are thinking of the early Zionists, who seriously considered adopting German as their language in the Holy Land, before Hebrew won out. More than you would want to know at https://www.osmikon.de/en/thematic-dossiers/shared-histories/translate-to-english-zur-rolle-des-deutschen-im-zionismus-bis-zur-balfour-deklaration