starting on i18n

First stab at i18n. This partially implements #4 and #5. Still to do:

 * UI to change languages (probably just on login page)
   * for now, you can switch languages with URL params (e.g. `?setLng=zh-HANT`)
     * this works on tasks and on the login page
 * filtering out tasks with unsupported languages
 * translating task and battery names