How to solve Portal/Tableau reports issues due to Windows's regional settings

Some users will have strange data behaviours when they extract reports from Tableau/Analytics/[PIAS] portal (these will all be impacted by Tableau so we will only refer to "Tableau" is this guide). This is typically due to a combinaison of settings in Tableau and Windows.

The symptoms are:

numbers that can't be added together
numbers that aren't correct (5678 becoming 5.678 or 5,678)
dates that aren't expressed as they should (US format for instance)

All of this is due to the fact that UK, US, France and Belgium aren't using the same notation for the thousands, the decimal or the dates:

UK would write "123,456,789.0" and "31-12-2018"
US would write "123,456,789.0" and "12-31-2018"
France would write "123 456 789,0" and "31-12-2018"
Belgium would write "123.456.789,0" and "31-12-2018"

There are 3 places where the settings could be adapted and all of these settings should be in line with each other if you want to avoid these odd behaviours:

The "Region" settings in the "Control Panel"
The "My Account Settings" in Tableau
Excel

The settings that work together

Settings that have been confirmed as working together

Country
Windows settings (Region/Format) Tableau settings (Locale) UK
English (United Kingdom) English (United Kingdom) US
English (United States) English (United States) EU in general
English (Belgium) French (Belgium) France
French (Belgium) French (Belgium) Belgium
French (Belgium) French (Belgium)
Netherlands
Dutch (Netherlands)
French (Belgium)

Where to edit the "Region" settings in Windows?

In the "Control Panel" you'll find the "Region" settings:
In the "Region" window you can define the "Format" you want to use:
Beware: Some users know about this setting and modify the additional settings in this section:
These are the typical settings you'll find for all number related settings:

English (Europe) OR French (Belgium)

English (United Kingdom)

English (United States)

French (France) -> switch to French (Belgium) if you have issues with all your settings on French (France).
These are the settings that will have an impact on the dates:

English (Europe)

French (Belgium)

English (United Kingdom)

English (United States)

French (France)

The "My Account Settings" in the web interface

Go to http://reports.pias.com:8000/#/projects and connect with the user's account
Click on the user's name in the top right corner and select "My Account Settings"
There you'll find the "Language" and "Locale" settings:
The "Language" setting have no impact on the reports formatting so don't worry about that.
The "Locale" however is the setting you'll want to harmonize with the one you have in Windows (see previous section).
As mentioned before, there are some Windows "Format" and Tableau's "Locale" that work well together. Some others are causing issues (for instance "French (France)" as Windows "Format" doesn't pair well with the "French (France)" option as Tableau's "Locale")
For our Dutch colleagues that select "Dutch (Belgium)" as Windows format and as Tableau Locale. It doesn't work. The solution remains to use "French (Belgium)" as Tableau locale.
Once you've decided which Locale you wanted to use, press the "Save Changes" button.

Excel

This last section is typically not somewhere you would want to modify the way it's done but if you happen to have modified these settings in the past, here is a short additional info on what to look for:

In Excel, there is also an option to make some French users happy after you've set the Windows "Format" to "French (Belgium)" so they'll have a space as a thousand separator.

To access this option follow these steps:

In the ribbon, click on "File"
Click on "Options"
In the "Excel Options" window that opens you'll have to select the "Advanced" tab and you'll find the separator settings.
Usually Excel uses the system separators (the ones defined in Windows (see the Windows section for more info about how and where to set this up) but you can uncheck the box and use a space character instead of a point (in the case of the below image).

/!\ IMPORTANT LAST STEP /!\

Once you have configured all those settings you should close Excel completely, refresh the webpage with the report and run + download the report again from scratch.

Additional considerations if you want to drill further into the topic:

With "French (France)" Format in Windows this is what happens when we modify the "Locale" in Tableau:

Language: French + Locale: English (UK): 48,773,939 --> This is the English format and isn't recognized as a number in Excel
Language: French + Locale: French (France): 48 773 939 --> Note that the spaces between the "48" and "773" isn’t the same ascii character as a normal space (at least with our French settings). The “standard” space is Alt+032, the one that’s there is Alt+0160 (Excel is very handy with its formula =Code(“ “) to figure that out).
Language: French + Locale: French (Belgium): 48.773.939 --> This works just fine. No idea why it works better that way than with "French (France)" on both sides.