Add This field is missing RU translation

<source>This field is missing.</source>
Have no translation. Not exists in EN version too. Just forgotten? 
Symfony\Component\Validator\Constraints\Collection()
public $missingFieldsMessage = 'This field is missing.';
merged branch fabpot/moved_validator_4 (PR #7784)

This PR was merged into the 2.1 branch.

Discussion
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Add This field is missing RU translation

This PR was submitted on the symfony/Validator read-only repository and moved automatically to the main Symfony repository (closes symfony/Validator#4).

<source>This field is missing.</source>
Have no translation. Not exists in EN version too. Just forgot? 
Symfony\Component\Validator\Constraints\Collection()
public $missingFieldsMessage = 'This field is missing.';

Commits
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97c4291 Add This field is missing RU translation