What does this PR change? What problem does it solve?

This PR modifies "layouts/partials/header.html" by adding site.Params.disableLangToggle to disable language switch toggle if set to true. (I think sometimes users may just want to provide translation functionality in blogs instead of the whole site 🤔)

For example, if set params.disableLangToggle to false in "hugo.yaml/toml/...", everything stays the same:

If set to true, then the language switch toggle will not appear, but the one in blogs will still available:

Was the change discussed in an issue or in the Discussions before?

This change is not discussed in an issue before, but is a simple yet lovely enhancement.

PR Checklist

• This change adds/updates translations and I have used the template present here.
• I have enabled maintainer edits for this PR.
• I have verified that the code works as described/as intended.
• This change adds a Social Icon which has a permissive license to use it.
• This change does not include any CDN resources/links.
• This change does not include any unrelated scripts such as bash and python scripts.
• This change updates the overridden internal templates from HUGO's repository.