feat: i18n support

[MomentQYC]

resolve #13
Initial implementation of i18n, the future will add the part of the user's independent choice of language.
I'm relatively new to js, so please point out any errors, thanks!

[lowercasename]

This is a superb feat, thank you so much so far! I'm hoping to have time to review this early this month.

[MomentQYC]

When I create a new event, there is an error:
Database error, please try again :( - ValidationError: start: Cast to date failed for value "Moment" (type Moment) at path "start", end: Cast to date failed for value "Moment" (type Moment) at path "end"
I don't know if this is related to the code I changed (after all, I know very little about js).

[lowercasename]

There's currently an issue with this implementation where nested Handlebars expressions don't get rendered, so this i18n string:

"hostedBy": "Hosted by {{eventData.hostName}}",

Comes out as:

Hosted by

I think the easiest way to get around this is to separate the internal Handlebars expressions from the translation strings, since it seems quite complex to render nested Handlebars expressions.

[lowercasename]

When I create a new event, there is an error: Database error, please try again :( - ValidationError: start: Cast to date failed for value "Moment" (type Moment) at path "start", end: Cast to date failed for value "Moment" (type Moment) at path "end" I don't know if this is related to the code I changed (after all, I know very little about js).

The issue here is that the event creation code expects the start and end date to be supplied in a very specific format, and that format only comes across as expected when the datepicker is set to the en locale.