[DataGrid] Add bgBG locale

[DanailH]

Fixes #777

This PR adds 'bgBG' locale OTB. Currently, if you want to use that locale you would need to provide it to the grid in the following way:

import { DataGrid, bgBG } from '@material-ui/data-grid'

...

<DataGrid localeText={bgBG.components.MuiDataGrid.defaultProps.localeText} />

The reason is that we want to make it work with createMuiTheme and this is the v5 structure https://material-ui.com/guides/localization/#locale-text - #984

In a separate PR I'll add the ability to set the locale using createMuiTheme because currently, we are missing an identifier for the grid, a.k.a MuiDataGrid.

PS: I found that I missed a couple of labels so I added them as well.

[oliviertassinari]

A side question, will the documentation be added (https://material-ui.com/components/data-grid/localization/#locale-text) when we work on #984?

[DanailH]

A side question, will the documentation be added (https://material-ui.com/components/data-grid/localization/#locale-text) when we work on #984?

Yes, I would like to make a demo with the theme I think it will be nice to showcase how it works.

As part of this ticket, I can extend the docs saying we have 1 or some OTB locales and point to GitHub for those who would like to contribute, what do you think @oliviertassinari?

[oliviertassinari]

@DanailH No specific preferences on having a live demo. I see equal pros & cons so why not.

Linking the folder where the locales are sounds like a great tradeoff. It removes the need to keep a table up to date as we do in for the core component (contributors often forget to update it).