[Firefox] Add more supported languages. Only EN locale is currently supported

[acornestean]

Description:
The extension files, more precisely the “_locales” folder shows that only EN is currently supported.
As per the spec doc, more languages are supposed to be supported, but currently only EN is.

Steps to reproduce:
A.

1. Load the Firefox Firefox Bridge v0.1 in the browser
2. The welcome page opens in a new tab
3. Change the preferred display language to another language, other than English, for example French, German or Spanish
4. Reload the welcome page
5. Notice that the welcome page is still displayed in English

B.

1. Install a Firefox browser with a different locale, other than English (EN-US), for example ES-ES.
2. Load the Firefox Firefox Bridge v0.1 in the browser
3. The welcome page opens in a new tab and is not localized to adhere to the browser locale.

Expected:
More supported languages should be added to adhere to the spec doc.

Actual:
The welcome page is only displayed in English regardless of the preferred display language or browser locale.