Short answer (tl;dr)

How it is we forget languages

The example above is a 'fragment' of a larger sentence, which is being used as a title for an article. More specifically, this fragment is a subordinate open interrogative clause. In other words it represents an information question that might occur inside a larger sentence. The title is short for a sentence such as

Discover how it is people forget languages

Main clause questions usually use subject-auxiliary inversion. They put the auxiliary verb (here the verb be, which some grammarians think of as a main verb) before the subject. However, subordinate interrogative clauses put the subject before any verbs, just like in a declarative clause.

The full story:

Open questions:

Open questions, sometimes called information questions usually take the form of open interrogative clauses. There are two main types depending on whether the clause is a main clause or a subordinate clause:

1. How was it made? (main clause)
2. I asked how it was made. (subordinate clause)

Main clause interrogatives are introduced by an interrogative word or phrase (who, where, why, how and so forth). They also involve subject-auxiliary inversion¹. In other words we put the auxiliary verb before the subject. If there's no auxiliary in the declarative sentence we use the dummy auxiliary do.

Subordinate open interrogative clauses, for example the bold clause in (2) are made exactly the same way as main clause interrogatives such as (1), apart from that they do not involve subject-auxiliary inversion. They do not put the auxiliary before the subject. It stays in its normal position. If there is no auxiliary there, we do not use the auxiliary do:

3. He called who?
4. Who did he call?
5. I wonder who he called

In (5) we see that there is no subject-auxiliary inversion in the subordinate clause, and therefore we do not need to use auxiliary do.

Subordinate interrogative clauses appear as parts of larger sentences. They can be the subjects or the complements of verbs, the complements of prepositions and carry out many other syntactic functions:

6. How it was made is unclear.
7. I don't know how it was made.
8. We are aware of how it was made.

Sometimes subordinate interrogatives represent the unrevealed answer to a question. This is what is happening in the examples above. Example (7), for instance, means 'I don't know the answer to the question "How was it made"'. However, other times subordinate interrogatives just represent questions as in example (2) and in example (9) below:

9. I wonder how it was made?

Note that the sentences that subordinate interrogatives occur in are not always questions themselves. All of the examples above occur inside declarative clauses. These declarative clauses usually, but not always, represent statements. However, also note that subordinate interrogative clauses also very often occur inside other questions, i.e. inside other interrogative clauses:

10. Do you know how it was made?
11. Who here knows how it was made?

And, they can also occur inside imperatives:

12. Ask how it was made!

And also inside exclamative ones:

13. How strange that she asked how it was made!

The Original Poster's question

The Original Poster asks why the subtitle for an article How it is we forget languages has the word it before is. Normal questions, after all, have is before it.

Good question! The answer is that it precedes is because this subtitle is a fragment. A 'fragment' is an incomplete part of a sentence. Fragments are very common both in informal speech and also in song titles, book titles, heading and subheadings.

More importantly for the Original Poster's question, the fragment:

14. How it is we forget languages

Is a subordinate interrogative clause. Because it takes the form of a subordinate clause, we do not see any subject-auxiliary version. If it was a main clause we would do.

We saw before that subordinate interrogative clauses can represent not only questions but also answers to questions. That is probably the best description of subordinate interrogative clauses used as titles. We cannot know what full sentence this fragment is from. It could be something like one of these:

15. Read below to find out how it is we forget languages!
16. This is about how it is we forget languages:
17. Everything you ever wanted to know about how it is we forget languages
18. Don't you want to know how it is we forget languages?

The Original Poster also asks if it is is an inversion. The answer is that it's probably best to think of it as not being an inversion. A normal main clause question involves an inversion. A subordinate interrogative doesn't reverse this inversion. It just does not have any inversion to begin with. The evidence for this is that we do not need auxiliary do in subordinate interrogatives, as we saw in example (5).

That's all folks!

Grammar Notes

^{1Main clause questions do not have subject-auxiliary inversion if the wh-word is the subject:}

• ^{Who ate the cookie?}

^{In the example above who is the subject of the verb ate and there is no auxiliary inversion.}

^{2 Most modern grammarians believe that BE is always an auxiliary verb, even when it doesn't have another verb afterwards. this makes the grammar much simpler. However, some traditional grammarians still believe that BE is only an auxiliary verb when it has another verb after it. Their grammar is more complicated.}