Thanks for your speed information.

System - Language Filter plugin is disabled => ok
JoomSEF language management enabled in JoomSEF's Configuration => ok


Please post an example of what URL is used
=> for example:

Real URL non sef in Joomsef database: index.php?option=com_rsform&formId=4&lang=en&view=rsform&Itemid=447
URL displayed when this sef url is with "no": /index.php?option=com_rsform&view=rsform&formId=4&Itemid=447&lang=en

It seems that ID are ok. With the second URL (sef: no) mainmenu is ok. What do you think about?

Yes, the Itemids look correctly. Are there any duplicates for the SEF URL? I mean, is the SEF URL present in JoomSEF's URLs Manager multiple times with different Real URLs?

Yes I have several URL between SEF URL and real url; with the same example:

fr/projet-d-achat-2/projet-dachat-de-machine-copy
=> index.php?option=com_rsform&formId=4&lang=en&view=rsform&Itemid=447

fr/projet-d-achat/projet-dachat-de-machine
=> index.php?option=com_rsform&formId=2&lang=fr&view=rsform&Itemid=319

projet-d-achat-2/projet-dachat-de-machine-copy
=> index.php?option=com_rsform&formId=4&view=rsform&Itemid=447

projet-d-achat/projet-dachat-de-machine
=> index.php?formId=2&Itemid=319

It's not good? This URL has been generated automaticilly

There seems to be some problem with the language code, for example your first URL:

fr/projet-d-achat-2/projet-dachat-de-machine-copy
=> index.php?option=com_rsform&formId=4&lang=en&view=rsform&Itemid=447

The language code in SEF URL is "fr" but the Real URL contains "lang=en", which shouldn't happen.

We'll check our RSForm extension whether there's some problem. Could you also send me access credentials to your website's backend by e-mail, so I could check your menus setup? The e-mail is Tato emailová adresa je chráněna před spamboty, abyste ji viděli, povolte JavaScript .

Hello,

Thanks.

I have sent you email.

Have a good day