Hi,

There are two possible ways to change the format of the date in the template.

1) You can change the format from the Languages that you have tried already.

2)The second method you need to inherit the actual report template and add the date additionally in it

a. You need to locate the actual report template of from the addons

b. Inherit the template in your custom module

c. Add custom date inside the template.

example:

You can change the format like,

<span t-esc="date.date().strftime('%d/%m/%Y')"/>

d. Run the module

Hope it helps