Fallback to default language if string isn't available in i18n

### aiogram version

3.x

### Problem

Currently, if for a language a string is missing, it shows the key of that string.

### Possible solution

Instead of showing the key of the string, to get the default language's string and use that instead.

### Alternatives

```python3
FSMI18nMiddleware(i18n := I18n(path="locales", default_locale="en")).setup(dp)
default_locale = i18n.locales[i18n.default_locale]
for locale in i18n.locales.values():
if locale is not default_locale:
locale.add_fallback(default_locale)
```

### Code example

```python3
# lang = en (default language)
text = _("message.hello") # Hello!

# lang = ru (not 100%d language)
text = _("message.hello") # message.hello
```

### Additional information

_No response_