I find nowhere to set this in the application/config.php, so where to set it? I can accept hacking the code. Thank you in advance!

Default by language : github.com/LimeSurvey/LimeSurvey/blob/40...lator_helper.php#L65

You can makoe a feature request to allow update this by DB, config or plugin

Thank you for your tip. But finally it turns out that the Import user from CSV feature ignores the "lang" field value in the CSV file and the dependent "dateformat" value. This is probably a bug in the latest 4.x version.

Token import ?

can you show the header and some data ? Remind : language for token must use language code ( fr, en, de …)