First, you need to decide which language you want to have by default.
The browsers actually never report set keyboard language, the browser reports only language of browser, where the program can assume, which language should be preferred.
For example if your browser is in French, then you get French as standard language, in German language you will get German and so on.
If you accidentally install English version of browser instead of your language, then you will get english as auto-recognized language.
Therefore to avoid this, you can force your language on the page Clients\www\index.html, as example for german:
var lang = 'de_de';
Remember, changing this variable inside Clients\www\software\html5.html will not affect the language, since this variable gets overwritten by parent window, where you opened this page from.
On your HTML5 installation inside Clients\www\software\html5\languages\ you will find known languages.
Inside RDP session you should switch to wished language, the program will then automatically switch the used translation table.
That means, if you set German inside RDP session, you have to switch to German on your local system to be able to type specific German chars.
If you set to English inside RDP session, then switch to English on your local system too, and vice versa.
If you notice some problems with keyboard input, please first check that the option in Clients\www\software\html5\settings.js
preferchars = true;
is really set to true and not false.
If you do not have a file named "settings.js" in the "C:\Program Files (x86)\TSplus\Clients\www\software\html5" folder, then your TSplus version is older and this documentation does not apply. Please update your system first or contact support.