Thank you for help and quick answer, burnout426.

I won't turn off date grouping because I think it is good.

As both the Opera mail and my OS are set to my local language, I guess there is nothing to do. Too bad

Thank you for additional information

I was looking for an easy way to handle multiple mail accounts in one place and discovered Opera Mail something like 2 weeks ago.

For now, I am fully satisfied with it, it appears to be a good solution.

There is nevertheless one thing that bothers me.
Opera is grouping my mails by date, and I noticed it starts a new week on Sunday (I suppose it is accurate for some countries but it is not for mine).

I'm looking for a setting to set first day of the week as Monday.
Where can I find it ? I'm sorry for the bad english.