In the last (at that moment) version of Lubuntu 19.04 desktop environment was moved to LXQT. In looks fine, but the role of screensaver still occupied by xscreensaver application with it's awful window to enter the credentials. I've already have a post about theming this window https://16rom.com/ru/blog/temizaciya-xsreensaver .
You should delete it immediately after OS installation:
sudo apt-get purge xscreensaver
My search was finished when I found simple and minimal i3lock. The main feature of this screensaver is absent fields to enter user credentials! Check it's website https://i3wm.org/i3lock/
And install it:
sudo apt-get install i3lock
Screenlock accepts typing the user password without changing it's locale. And this is the main problem - if your password consist of Latin symbols, but when the screenlock was launched you had another locale (Cyrillic for example) than enter the password become an unresolvable task!
To prevent this issue I wrote a simple script in file /home/user/screensaver.sh
#!/bin/bash xkb-switch -s us i3lock -t -i /home/user/Pictures/screensaver.png
You should install xkb-switch application to change the locale, because standard xsetkbmap it's not so comfortable due to the resetting of all installed locales.
Source code for xkb-switch located here: https://github.com/ierton/xkb-switch
Locales switches I organized with the script added to the ~/.bashrc file
setxkbmap -option "grp:switch,grp:caps_toggle" -layout "us,ru,ua"
Hotkeys for Locale switch you can find in menu:
Menu -> Preferences -> LXQt settings -> Shortcut Keys
For Hotkeys Ctrl+Alt+L I set command to launch the script from the file /home/user/screensaver.sh