Learn English – Usage of that: There are many reasons [that] I am confident [that] I will excel at this job

Yes, you probably are losing your mind. :)

I say that (in jest!) because there is no grammatical or spelling mistake in this sentence:

There are many Danes who speak English.

Depending on the writing assignment, they may wish it rewritten for any number of reasons, but the spelling and grammar are still ok.

They might conceivably want something like these:

• Many Danes speak English.
• There are many Danes who speak English well.
• Most young Danes speak English well.
• Most young Danes today have quite good English.

But those are all just elaborating and rewriting to emphasize different aspects of the thought they wish to convey. They are no better in terms of grammar or spelling than your original.

John Lawler in comments points out two possible things they might be thinking are wrong.

The use of many instead of a lot of is a register difference, but could only be considered ungrammatical by someone who believed that many and much are pure negative polarity items. They do display some negative preferences but they can certainly occur outside a negative context, and often do.

So one is that many is of a different, less casual register than a lot of.

• There are a lot of Danes who speak English.
• A lot of Danes speak English.

Those are more casual than the original, but mean the same thing.

The other issue John raises is that they might be thinking that many is automatically limited to negative contexts only, such as:

• Not many Danes speak English.
• There aren’t many Danes who speak in English.

However many isn’t so limited. It may also not be true. :)

There are places where many can only be used as a negative polarity item, but this is not one of them. See Professor Lawler’s NPI cheat-sheet.

If you wanted a version of your original that did have an error in grammar or spelling, then any of these would do:

• *There are many Danes who speaks English.
• *There is many Danes who speaks English.
• *There are many danes who speak english.

One thing that would not be an error is the who. You can say who there just like you can say that (some people might even try to say which), but that doesn’t change anything.