Add @mui/lab/locale

When using localization, the theme can be extended with

import { createTheme, responsiveFontSizes } from '@mui/material/styles';
import { frFR } from '@mui/material/locale';
import merge from 'deepmerge';

// localized theme
const theme = merge(responsiveFontSizes(createTheme(themePreset), frFR);

However, components inside @mui/lab aren't localized.

• I have searched the issues of this repository and believe that this is not a duplicate.

Summary 💡

Examples 🌈

With @mui/lab/locale, extending the theme would decouple de app's responsibility to localize these components.

import { createTheme, responsiveFontSizes } from '@mui/material/styles';
import { frFR } from '@mui/material/locale';
import { frFR as frFR_lab } from '@mui/lab/locale';  // suggested
import merge from 'deepmerge';

// fully localized theme
const theme = merge.all([ responsiveFontSizes(createTheme(themePreset), frFR, frFR_lab ]);

Motivation 🔦

Components like TimePicker aren't localized. Once they are, and they are moved to @mui/material, then their localizations will move with them. User code should not maintain these localization strings.

The proposed entry point is @mui/lab/locale, with Lab components such as TimePicker needing localization alongside @mui/material/locale. Start by tracing the existing material locale setup and the Lab component localization data; done means Lab locale exports can be merged into a theme without user-maintained strings.