Facebook scraper ignores default language
Without knowing exactly how the following tags are generated I cannot provide the exact solution to your issue, but only to point out what causes the Facebook to grab EN version. 

1. &lt;link href="https://www.example.com/node/581?lang=en" rel="alternate"
&gt; hreflang="en" /&gt;

2. &lt;link href="https://www.example.com/task/581"
&gt; rel="alternate" hreflang="de" /&gt;


Tags 1 &amp; 2 have rel attribute set to alternative, which tells facebook that both links from tag 1 &amp; 2 can be used for fetching info. Therefore it grabs the first one in line.

Each page should have rel="canonical" tag that will tell facebook and all other crawlers, that it is a primary url for that page that should be used for fetching info.

I do not know if you have a link tag with rel="canonical" in your source code, as you only copied a portion of your meta tags. I suggest you check if it is present, and what page version it points to. If it is not present, add it.

Also, meta tag og:url should point to the right version of the page.

&lt;meta property="og:url" content="https://www.example.com/task/581" /&gt;


Test your tags by fetching new meta info via Facebook Open Graph Debugger.