Problem: plural forms for strings

### Current Behavior

**Steps to reproduce the behavior**
Currently ther are separate strings for the "singular" and the "plural" strings.
Examples:
1.
`%{smart_count} file selected`
`%{smart_count} files selected`
2.
`%{complete} of %{smart_count} file uploaded`
`%{complete} of %{smart_count} files uploaded`

### Expected Behavior

Strings which need "plural" forms should be set as one source string with language dependent "plural" forms. Many languages need more than just one plural form (for instance Croatian, Slovenian, etc.)

### Possible Solution

_No response_

### Context and Notes

_No response_

### Version used

_No response_

### Operating System and version

_No response_

### Default installation culture

_No response_

### PHP version

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### Contact details

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The issue does not name any files, tests, or entry points. Start by locating where singular and plural strings are currently defined and selected, then determine how one source string could express language-dependent plural forms; done means supporting languages that require more than two forms.

Written by the indexing model from the issue text.