Why the words used in devise are not always translated...?

in devise/sessions/new.html.erb I have this

<%= hidden_field_tag :from, @from %>
<!-- Username -->
<div>
    <div><%= f.label :username_or_email %></div>
    <%= f.text_field :username_or_email, autofocus: true, required: true %>
</div>

<!-- Password -->
<div>
    <div><%= f.label :password %></div>
    <%= f.password_field :password, autocomplete: "off", required: true %>
</div>

But I have a problem with the translation. I don't know why but "password" word sometimes is translated (for example in Japanese) but sometimes not (for example in Chinese).

(The word "password" is not added in my config/locales/ja.yml even in config/locales/zh.yml)

While username_or_email is never translated. Why? Should I download those files? Now it works also without them for several words (i don't know how...)