The language code is useful only when you are going to share your report with other colleagues who live in other countries. As you know, Germany, Canada, Japan, Argentina, US, France and others have a different version even if the language is in English.

When I said version I mean Japanese version, US version, Russian version, Italian version, etc. Generally, in big companies, most but not all installed versions are using English, the others are using the local language.

So if other colleagues from the same country is using your file, there is no issue to work with the date but for others in other country, it will be a problem depending of the tasks. For instance, sharing an excel file between Spain and Germany, the date is incompatible.

It is not a problem to format the cell by showing the date differently. For instance, in Europe, the date is DD/MM/YYYY and in US, it is MM/DD/YYYY so you can format the cell as you want, you will not see an issue, the issue will appear when your US colleagues will use the same file as you.

In such situation, you will need to include the language code. For instance, for US, the code is 409. Well in the Microsoft page, it will show 0x0409 so formatting the cell, you have to put:

[$-409]dd/mm/yyyy

So if your colleagues from other countries will tell you that they got an error because of the date and not you, use the language code.

To known the correct code for each country, go to the official Microsoft language code webpage.