Decimal Numbers Interpreted as NULL in Tableau Desktop

Issue

When connecting to a text file decimal numbers are not interpreted correctly. Trying to change the data type results in the numbers becoming NULL

Environment

Tableau Desktop
Text Files

Resolution

To make Tableau recognize all the dots as decimal separators and recognize the values as Decimal numbers by default, follow the step below:

Connect to the .txt file.
On the data source page, click the small arrow displaying on top of .txt file on the Data pane in Tableau and select "Text file properties". (This option is for text files. It will not be available for Excel or other formats.)
In the dialog menu change the Locale of the file to English.

Cause

This issue often occurs because a dot (.) is used as decimal separator, but Tableau uses a non-English locale by default. The other way around is also possible, a comma (,) is used as decimal separator, but English locale is set in Tableau.