I have not studied the entire contents of this thread, so you may have already tried this:
I don't know if it will help. I and my family live in Denmark, but my language of choice is English.
I have always installed with the Danish OS, for those family reasons.
For the computers which I personally use, I installed the English (UK) language pack. It was necessary to keep the Keyboard and Date, time and number formatting in Danish , however.:
In the Advanced settings of the Control Panel - Language, I overrode default and chose English(UK) in both bars.
At the bottom of the same window, I ticked the option "Don't let websites....etc
Back in the Control panel, under regions, I clicked in English (United Kingdom)
This solution has always worked for me. All my content, when online or installing software, is in English.