Allow passing locale=FI/SV/EN

[JK0N]

User can change the language when page is loaded but it doesn't seem to handle this param on initial load?

For example when you open https://opas.matka.fi/?locale=sv ... It still seems to render Finnish version on my browser.

[vesameskanen]

Language selection is not documented well. You can do it by adding fi, sv or en to the url: https://opas.matka.fi/en