I tested this on Linux Mint Cinnamon 22.2 with 2 different ways.
For the first way, I went in the start menu, right-clicked on the Opera shortcut and chose "Add to desktop". Then, I moved that opera.desktop file from my desktop to "~/.config/autostart". I then, right-clicked on it, went to properties and changed the command to opera --lang=fr %U and that worked. When I logged in, Opera started automatically and in French.
For the second way, in "~/.config/autostart", I created the opera.desktop file in Text Editor with the following content:
[Desktop Entry] Version=1.0 Name=Opera GenericName=Web browser Comment=Fast and secure web browser TryExec=opera Exec=opera --lang=fr %U Terminal=false Icon=opera Type=Application Categories=Network;WebBrowser; MimeType=text/html;text/xml;application/xhtml_xml;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/ftp;application/x-opera-download; Actions=new-window;new-private-window; [Desktop Action new-window] Name=New Window Exec=opera --new-window TargetEnvironment=Unity [Desktop Action new-private-window] Name=New Private Window Exec=opera --incognito TargetEnvironment=UnityAfter the file was created, I right-clicked on it, went to "Properties", switched to the "Permissions" tab and checked "Allow to execute this as a program" and that worked. When I logged in, Opera started automatically and in French.
