E.g. from December 2015 to January 2016, there can be different starts of the week numbering in different countries.
In Austria, week number one is the “first 4-day week”, which is similar to Germany.
But if you use any .Net software or have written one by yourself, you will see the week number is calculated incorrectly having set “German (Austria)” instead of “German (Germany)” in your Windows region settings.
From 2015 to 2016, we have a week number 53 which ends on Sunday, 2015-12-29. The following week which starts on Monday, 2015-12-30 should be week number 1. The “Austria” setting shows week 53 and the next one is 2, which is wrong.
As it is hardly possible to get a response from Microsoft regarding this issue, the only workaround I found so far is to set your region to “German (Germany)” on all your Windows stations. Fortunately, this can be done in the entire LAN via Domain GPO setting.
Hint: If you wonder why the new region setting is not saved in your GPO, use F6 key to turn the underline from red to green.