I already discovered the historian Christopher Clark some years ago when watching the documentary series “Die Deutsche Saga”, thatwhich narrates in six chapters the origin and the evolution of the German nation.

Use which instead of that
you don't need to repeat "the"

I remember fouinding it very interesting, not leajust because of the charming voice and the marvellous storytelling of the presenter, the Australian historian Christopher Clark.

Either say "I found it very interesting" or "I remember finding it very interesting"

Clark is a professor at Oxford University, who specialized in the history of Prussia and the First World War.

I found his knowledge of the German language astonishingly good.

I think, that they are very interesting and funny.

We shouldn’t permit that his role as a presenter to obscures the his role as a historian.

Therefore, his works are well worth a reading.

I had already discovered the historian Christopher Clark some years ago when watching the documentary series, “Die Deutsche Saga”, that narratesd in six chapters the origin and the evolution of the German nation.

Here we want to use the past perfect, to emphasize that this action occurred in the past a long time ago; therefore, "I had already discovered..."
Remember to check your tenses when you write

I remember fouinding it very interesting, not least because of the charming voice and the marvellous storytelling of the presenter, the Australian historian Christopher Clark., but because of the intricacy of the story itself.

FOUNDING is for when you establish a company, country, etc
FINDING is the gerund form of find

"not because of ... but because of ...."
So your phrase here is missing an idea

Clark is, a professor at Oxford University, specialized in the history of Prussia and the First World War.

He is married to a German woman.

I would delete this sentence as this doesn't connect to the preceding or the following sentence.

I found his knowledge of the German language astonishingly gooadvanced.

This sentence is correct but in writing, especially at an upper intermediate level, we need to avoid using adjectives like "good" or "bad"

He iwas the host, not only of “Die Deutsche Saga”, but also of “Die Welt Saga”, both of themwhich were streamed on the German TV channel ZDF.

I think, that they are very interesting and funny.

We shouldn’t permit that his roldiscount his competence as a historian just because he was a presenter for many obscures the historian.elevision shows.

sorry for changing your sentence drastically but that sentence construction was very strange

Therefore, his works are well worth a reading.

AsSome good examples we can mentioninclude “Iron Kingdom: The Raise and Downfall of Prussia” or “The Sleepwalkers”.

You can find a couple of speeches related to thoese topicitles on the internet, normusually in English.

normally is not used like in French
For English, normally is not used like usually
topics is too wide, you mentioned two specific pieces of work

Nevertheless, I have to admit that I discovered him in German, and this is the tolanguage that I automatically link to himassociate with his work.

tongue is the organ that we use to taste food, not a language

I would delete this sentence as this doesn't add any additional information that doesn't get conveyed by the following sentence.

He speaks several languages to some extentwith varying degrees of fluency.

I would delete this sentence as this doesn`t add any additional information that doesn't get conveyed by the following sentence.

To the already mentioned English and German, we should add French, Italian, Spanish and Serbo-Croatian, being the latter of which being important in his work on the First World War, twhenre the Balkans played a crucial role in the conflict.

Move this sentence up to where you described his linguistic skills