I use two languages (English & Greek) and I use flags to know which language I am writing to. There isn’t an indicator at all. Do you know how to install one?
How to add flags to indicate languages on a panel
Which DE are you using?
Some of the xfce panel plug-ins have to be installed separately, I believe what you are looking for is:
It’s in the repos.
Then right click panel > add new items > keyboard layouts