Support i18next keys that do not match the source file

### Describe the problem

Currently there is no possibility to define units that do not match the source file while using monolingual localization framework. This feature could be used to support word cases which are different for every language inside glossary.

For example:

```
// ru.json
{
"word-nominative": "слово",
"word-genitive": "слова",
"word-dative": "слову",
"word-accusative": "слово",
"word-instrumental": "словом",
"word-prepositional": "слове",
}

// en.json
{
"word": "word",
}
```

### Describe the solution you would like

Enhance multistrings not only with plurals, but also with cases: `"word_one#nominative": "слово"`.

### Describe alternatives you have considered

Allow manually configure component not to drop units that don't match the source file.

### Screenshots

_No response_

### Additional context

_No response_