HTML5 client erroneously fetches keyboard layouts from the browser's language

[Carlos Solís]

When connecting to Xpra from the HTML5 client, the system attempts to obtain the keyboard layout from the browser's reported language - unfortunately, in my particular case this is causing wrong guesses, namely:

• My Firefox browser reports "ES-ar" (Spanish, Argentina) and Xpra's logs report switching to "ar" (Arabic).
• My Microsoft Edge Browser reports more accurately as "ES-cr" (Spanish, Costa Rica) and Xpra's logs report attempting to switch to a non-existing "cr" layout, causing a failure.

In both cases, this causes the inability to type into any windows. Xterm in particular only seems to display numbers and a few control characters. Not even setting the server to use a specific keyboard layout through the --keyboard-layout=XX command seems to work, as the layout is dynamically switched because of the heuristics detailed above. This bug might have gone unnoticed before, because there are EN and US layouts available by default.