feat: translate collection labels

[razonyang]

Let's say there is a collection:

params:
  decap_cms:
    collections:
      hello:
        label: Hello

To translate the Hello in other languages, such as Simplified Chinese (zh-hans), create i18n/zh-hans.toml and use cms_label_hello (lowercase) as the key.

[cms_label_hello]
other = "你好"