We have 3 windows installs, two desktops running windows 10 in virtual box and one laptop with only win-10 installed, all 3 have an installed language of English (United States) are running office 365 family. All 3 exhibit the proofing problem.
In office 365 family I have added German (and set it as preferred) in the office authoring languages and proofing popup window from View>Language. I have left the Office display language as match Microsoft windows [English] <preferred>.
In power-point>options>Language>office authoring languages and proofing I have German (Germany) <preferred> and the office display language is English.
In power-point>options>proofing>custom dictionaries, I don't have a German option so I have All languages>roamingcustom.dic (default) and CUSTOM.DIC selected.
When I open a new slide or project the notes and slide are apparently being proofed in German (i.e. there are no spelling error wriggly red lines under the text) and the “mark selected text as: ” setting (bottom left corner of the window) is set to German.
The problem is that whenever I try to edit slides and-or add notes the software auto reverts to English (united states) and starts auto correcting the spelling e.g. if I type “moderne” and then space, power-point removes the “e” leaving me with “modern”.
Any ideas what I have done wrong and-or missed?