By default the HTML5 client tries to recognize browsers default language and then use it inside RDP session. This behavior is controlled by tag "asbrowser" However you may find the full list of available language tags in http://***yourserver***/software/language.html under "Locale"
You have the choice to set fixed language either A: per client directly from HTML5 top menu.
B: or from portal by setting it in AdminTool GUI which will be effective for everyone.
You should restart session if setting another language as main.
Remember: when using HTML5 client in mulitple language scenario go sure that language used inside Web-HTML5-RDP session is synchronized with the language actually set on your PC/Laptop/Mac etc. Else some chars that are represented and crossing in every language map may produce wrong char by sending scan code actual for another language. Also that means, if you changed the language from English to German inside web-session then change it to German on PC/Laptop/Mac too.