Macro extend 

macro_rules! extend {
    ($target:ident) => { ... };
}

Extend a dependency’s translations with the matching crate namespace from the current crate’s backend.

Given extend!(ui_component), translations below the ui_component key in the current crate are lazily merged with translations in the ui_component crate. The extension takes priority for the same locale and key.

The lookup order for each locale and key is equivalent to:

app.translate(key)
    .or_else(|| ui_component.translate(key))

If both miss, the existing locale fallback rules continue as usual.