Improve strings in language select

[vjik]

I find it somewhat inconsistent. Mb it'd be worth picking out common keys into, say, messages/common

return [
    'layout.language.english' => 'English',
    'layout.language.russian' => 'Русский',
    'layout.language.slovak' => 'Slovenský',
    'layout.language.indonesian' => 'Bahasa',
];

Originally posted by @alamagus in https://github.com/yiisoft/demo/issues/492#issuecomment-1212747144