launcher: set preferred language list from system locale

[rawoul]

This changes the Accept-Language header to match the system locale by default.

[rawoul]

Thanks for the review, I've fixed the issues you raised and factored the code in a separate function for clarity.

[TingPing]

I have one concern, this exposes system information directly to the web which adds information for fingerprinting. Browsers like Safari specifically filter and reduce this list to make it less unique.

If this does or doesn't matter to Cog is probably for another dev to decide.