Notes

(1) According to the phonetic transcription from Wikipedia, it's [ˈʃaːndor ˈmaːrɒi], hence Шандор Мараи

(2) No need to capitalise the names of the months in Russian. 

(3) I'm not 100% sure about it, but I know that the Hungarian word ország is pronounced [ˈorsaːɡ]. I think Országh should be pronounced in a similar way. When transliterating names, it's best to pay attention to what they really sound like or follow some transliteration conventions.

(4) It's one of the phonetic rules: before some consonant clusters, especially those starting with a similar consonant sound, the preposition в becomes во. By similar I mean that the sound has more or less the same place of articulation: /v/ and /f/ are both articulated with the lower lip and the upper teeth, so it's more difficult to pronounce them one after the other. This is why we should add a vowel sound between them. 

(5) Not a mistake, but I would use a perfective verb here. Собирался начать ~ was going to start, собирался начинать ~ something like "was going to start starting". The latter doesn't sound wrong in Russian, but in this case the imperfective verb начинать emphasises that it's not that he just wanted to write in German, but that he was going to start writing in German (and then maybe to switch to Hungarian or some other language, for example)

(6) Grammatically correct, but it doesn't make too much sense in Russian, I'm afraid. I think it would be better to phrase it as in the Wikipedia article, so it would be something like this:

Для Мараи родной язык тесно связан с самим понятием нации.

(7) There are two things worth mentioning here:

    (a) "Потому как" = "потому что" = because; "по тому, как" ~ by (because of) the way how. So, ", потому как он писал в ясном реалистическом стиле" = "because he wrote in a clear realist style"; "по тому, как он писал: ясным реалистическим стилем" = "by the way in which he wrote: in a clear realist style". Something like that. 

    (b) Ясный реалистический стиль doesn't sound absolutely correct to my ear. I think it makes sense to change реалистический стиль into стиль реализма and maybe to use the other adjective you can see in the article:

...где он быстро стал известен, потому как писал в стиле реализма, ясно и точно. 

It's just a suggestion, though.

(8) Again, this is just how you write the name in Russian (and you need to use the genitive case here, by the way).

Very well translated! It's very nice to see that you write some things without relying too much on a dictionary, it's a good sign. Keep up the good work! :)