[I10n] Add Finnish (fi-FI) locale to pickers

[PetroSilenius]

I noticed that the DataGrid had support for Finnish locale but datepicker was lacking it

• Adds Finnish locale to Datepicker
• Adds missing Finnish translations to DataGrid

Translations were formed by myself a native Finnish speaker with the assist of Microsoft language portal and Google Translate

Couldn't find where to change the default placeholder for the datepicker. It should be pp.kk.vvvv in Finnish instead of dd.mm.yyyy

[mui-bot]

These are the results for the performance tests:

Test case	Unit	Min	Max	Median	Mean	σ
Filter 100k rows	ms	526	1,072.7	614.3	775.26	197.122
Sort 100k rows	ms	628.3	1,089.4	628.3	913.54	157.416
Select 100k rows	ms	209.8	289.6	221	242.06	33.512
Deselect 100k rows	ms	142.3	222.2	217.5	201.44	30.025

Generated by 🚫 dangerJS against 3b77235

[flaviendelangle]

Couldn't find where to change the default placeholder for the datepicker. It should be pp.kk.vvvv in Finnish instead of dd.mm.yyyy

For now the placeholder is not customizable with the locale.
That's something we are discussing for the next field component.

You can manually override the placeholder passed to your input like so:

<DatePicker
    label="Basic example"
    value={value}
    onChange={(newValue) => {
        setValue(newValue);
    }}
    renderInput={(params) => 
      <TextField 
        {...params} 
        inputProps={{ ...params.inputProps, placeholder: 'pp.kk.vvvv' }} 
      />
    }
/>

Note that it won't react to any change in the format and you have to manually take care of it.
For instance if you have a picker with only a year view, you will have to manually write vvvv

[PetroSilenius]

For now the placeholder is not customizable with the locale. That's something we are discussing for the next field component.

You can manually override the placeholder passed to your input like so:

Right, I assumed so since I wasn't able to find anything related to it. Sounds like a good addition as other translations are handled by mui that the placeholder would be handled also👍🏻

[PetroSilenius]

Would you be able to review this @flaviendelangle or how should I proceed with the PR?

[flaviendelangle]

Thanks for your contribution !

It currently targets the next branch which is for the v6 alpha releases.
I will port this PR to the master branch to have it available on the v5 release as well.