[Validator] Added Polish translation for the ISSN validator.
merged branch jakzal/validator-pl-trans (PR #7795)

This PR was merged into the master branch.

Discussion
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[Validator] Added Polish translation for the ISSN validator

| Q | 	A
| ------------- | ---
| Bug fix?	| no
| New feature?	| no
| BC breaks?	| no
| Deprecations?	| no
| Tests pass?	| yes
| Fixed tickets	|
| License | 	MIT
| Doc PR	| ~

Commits
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421bd12 [Validator] Added Polish translation for the ISSN validator.