Macro erg_common::switch_lang

macro_rules! switch_lang {
    (
        $should_english: literal => $msg: expr,
    ) => { ... };
    (
        $lang_name: literal => $msg: expr,
        $($rest_lang_name: literal => $rest_msg: expr,)+
    ) => { ... };
}

More languages will be added … マクロはパラメータを展開しないので、format!のロスがなくなる