{lang} variable not working inside custom collection link

[passatgt]

Expected behaviour

The following code should generate category archive pages for every language:
localhost:4567/en/category
localhost:4567/hu/category

Actual behaviour

The {lang} variable is not parsed at blog.custom_collections.category.link, so i get category archives without the proper locale

Steps to reproduce the problem

activate :blog do |blog|
  blog.sources = "articles/{lang}/{year}-{month}-{day}-{title}.html"
  blog.permalink = "{lang}/articles/{category}/{title}.html"
  blog.layout = "article"
  blog.custom_collections = {
    category: {
      link: '{lang}/{category}.html',
      template: 'category.html'
    }
  }
end

With two files located in:
source/articles/en/2017-01-01-title-en.html.markdown
source/articles/hu/2017-01-01-title-hu.html.markdown

And both files include category:Something in frontmatter

Additional information

• Ruby version: ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-darwin15]
• Middleman version: ~> 4.2
• Middleman Blog version: 4.0.2
• OS version: macOS 10.12.4

[markets]

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