WIP: Add Esperanto translation

[jorgesumle]

Esperanto is not related to any particular country, so I chose xxx as country code (I could write instead something like and, Andorra, I don't mind). The existing flag images are bigger than the README recommends, so I guess that the README should be updated:

Add a relevant flag to assets/images/flags/XXXX.png, replacing XXXX with the 3 letters localeCountryCode. The image must be 42x26 pixels with 3 pixels of transparent margin on all 4 sides. You can resize the flag with paint.net to 36x20 pixels, expand the canvas to 42x26 pixels with the flag anchored in the middle, and then manually delete the 3 pixels on each side to make it transparent. Or you can use another program like Photoshop.

They are 96x96 pixels.

[apertamono]

For the localeCountryCode, I'd recommend using the 3-letter language code from ISO 639-2: epo.

Good luck with your translation, and thanks for the effort!