Why is 'Easy' Isolde German?

Easy's father owns the lodge at Lake Wakapoogee and Abigail is the director of public relations.
THIS IS MY FAVORITE MOVIE OF ALL TIME!!!!

She fit the role.

Since Paula Prentiss wasn't considered a top star (Paramount refused to cast her, so Hawks went to Universal) they had to be careful not to cast someone as her sidekick who would steal her the show. And Austrian actress Maria Perschy was a good choice. She was just as delightful to watch as Prentiss was (especially in the scene where they both wear diving suits). But she is a cool restrained blonde and therefore a good backdrop for the more emotional and extrovert Prentiss. Hitchcock would have preferred Perschy, but everybody else knows that Prentiss will get Hudson, even when it is Perschy who is twice caught by Hudson's fiancee with Hudson jammed in a zipper.

I guess the writer's idea for Perschy's role was: in order to make Prentiss not too unlikeable for the audience they needed Perschy for the worst stuff. She gets rid of Hudson's fiancee. She is always inciting her friend ("Mach weiter, Du hast ihn gleich soweit", which means "Go on, you almost have him where you want him"). And doing that by speaking German will make the audience not understand the words, but understand her cool and more calculating character. And therefore understand that Prentiss' chaotic character is not cool and calculating, but warm, emotional, vulnerable and therefore likeable. Hudson understands that in the end, just as everyone else understands that "fishing" is not a matter of perfect planning and know-how but of sincerity and luck. (Well, the audience understands that, the business people around Hudson blame it on the equipment, but that's the irony.)