fixed string duplication for defaulted values

[Baptistemontan]

When a key was missing the value was defaulted to the default locale, but the process was to simply clone the value.
This worked perfectly but introduce some duplicates strings, now the value is not cloned anymore but marked to be defaulted to another locale, this basically change

match locale {
    Locale::en => "some string",
    Locale::fr =>  "some string", // <- duplicated string
}

to

match locale {
    Locale::en | Locale::fr => "some string",
}

[Baptistemontan]

This change also allows to take the target default, for now it is still always the default locale, but this could allow to specify different default for each locales, such that en-GB default be en, and fr-CA be fr, tho this could introduce recursives default, either infinite or finite, this is to take into account.