Resolve "Internationalization with Developer-Controlled Language Setting"

Related to #95

Since this is pretty much code I suggest:

• just go through src/i18n/en-US.json and check if the structure of key names is okay
• Check Internationalization (i18n) Guidelines in CONTRIBUTING.md
• Test app if it still works normal (at the moment there is no language switch planned, but i put in a hidden switch, which can be activated by clicking the version number in sidebar 5 times.
• Check commits:
  • !61 (b2c06667)
  • !61 (a8e6be84)
  • !61 (0eaf995d)
  • !61 (f5e01bf9)
  • Nice to have (would be nice to get input if you see other options to do that, but not mandatory):
    • !61 (a800e232)
    • !61 (8f93704e)

Know Issues

• Map is not translated
• Charts (ApexCharts) on Report page doesn't change dynamily