The international standard date notation is now
YYYY-MM-DD == iso8601
https://en.wikipedia.org/wiki/ISO_8601 largest to smallest e.g. year-month-day 2019-11-11
My (European, Irish) current(and for past 30 years) "normal" date notation is DD/MM/YYYY. https://en.wikipedia.org/wiki/Date_and_time_notation_in_Ireland
To make dates "normal" for me in DD/MM/YYYY format you have to check these three things:
in your Drive settings Language - Change language settings - e.g. English Ireland
This will set it for all newly created docs.
You only have to do this once.
in Spreadsheet settings (if spreadsheet was created different language), change Locale - e.g. Ireland
in Spreadsheet if some dates already there they might have custom format US style or no style so individually change those, e.g. select
- Format - Number - Date
- Format - Number - Date and Time
- Format - Number - More Formats - More date and time formats - ...
You have to check 2. and 3. for any older Spreadsheet or any Spreadsheet shared with you that might have US locale set or have individual US format date cells.
So, to make sure you are using date format iso8601(2014 or later I guess) in all your sheets you have to check the same 3 things but using Language setting, Locale and date format which has iso8601 as a standard, e.g. Polish, Hungarian, Canada, Sweden, . . . see @user135384 's list of options Change Google Sheets default date format with other locale on other answer.
This list of options will probably change over time as(or if!) iso8601 is adopted or if Google add formats e.g. locale English with iso8601.