The first sentence is usually the main point of a paragraph. The rest of the sentences support that main point.
In your paragraph your first sentence is fine grammatically but your support sentences are on a completely different subject - whether language should be taught at an early age. If you removed the first sentence all together then you have a good main point in the first sentence and the following statements support that statement.
I would make a small change to the wording of the second sentence.
Whether to teach foreign languages in primary or secondary schools has been a highly controversial topic for years.
Sometimes you have a tendency to create very long sentences and the third sentence is an example of this. When one sentence has more than 3 clauses in it, you should consider breaking that sentence into two or more sentences. A reader of such a long sentence usually must read it several times to find out what you are really trying to say.
While some people believe that a foreign language needs to be learned in an early age, for example in a primary school, others argue that it is too early for children at a very young age to be forced to learn new languages as well as to learn their mother tongue simultaneously.
I would break up this sentence as follows.
Some people believe that a foreign language should be taught at an early age, for example in a primary school.Others argue that this is too early for children of a very young age. They believe it is too difficult for young children to simultaneously learn new languages along with their mother tongue.
I changed "as well as" to "along with" which goes better with "simultaneously" in my opinion.
I moved the word "simultaneously" from the end of the sentence to split the infinitive " to learn". The use of split infinitives used to be considered a grammatical error, especially when I was in grade school. But that rule is relaxed now. If the sentence flows well with the split infinitive then there is no harm.
Consider the following two sentences:
"I hate him so much. I am going to hit him really hard when I see him.
"I hate him so much. I am going to really hit him hard when I see him.
These two sentences on the surface seem to mean the same but there is a subtle difference.
In the first sentence "really" is an adjective describing a harder than normal punch.
In the second sentence "really" is an adverb showing a strong desire to hit him.
So when looking at whether or not you wish to split an infinitive you should look carefully at how it may affect the meaning of the sentence.
On a personal note, I believe young children pick up languages much easier than older children or adults. Young minds are like empty sponges that soak up information quickly, especially if it is fun.
