This is neat!

Bug report: I uploaded an .osm file and a .gpx file, then clicked on filter on the left panel, the interface gets stuck at that screen when I click another panel and JS error "TypeError: legend is undefined" in Firefox

This is my first ever web application, and still in development. Any and all comments and tips are welcome, especially about the structure, API and other code-related aspects.

You should try to recognize the language of the user. I got everything in Finnish on my first visit. Then I chose English. On the next visit everything was Finnish again.