Nihongo o benkyou suru (better: shiteiru / shiteimasu) 

(Nanika)omoshiroi koto ga arimasu.

Nihongo wo benkyou suru. --> Nihongo wo benkyou shiteiru / shiteimasu. (I study Japanese.) 

Furansugo to supeingo to, doitsugo to wo benkyou shimashita.

Ima, Nihongo o hanamasu toki ni, doitsugo no kotaba, speingo no kotoba, furansugo no kotobo o oboeteimasu

--> Nihongo wo hanasu toki ni doitsugo ya supeingo ya furansugo wo omoidashi masu. 

(When I speak Japanese, I remember German, Spanish and French.) 

watashi wa yoku furansugo o hanashimashita. (I often spoke French.) 

Dakara, sorera no kotabo motomo kantan o obeteimasu.

(I am trying to say: Thus, those words should be easiest to remember.) 

--> Dakara, sorerano kotoba wa oboeru no ga kantan desu.

Demo, tokidoki watashi wa doitusgo no kotaba to supeingo no kotoba , hajimen (hitatsu??) oboeteimasu.

(I need the English sentence.) 

Watashi wa kono yō (you) ni kangaerimasu.

Yo milch nomimasu. (I drink milk). (--> Ah, you mix up 3 languages? I understand you!)

Good job! I'd like to suggest you not to mix up the formal way to say (desu/masu) and informal one (dictionary form) in one note though we do that in colloquial. Since I wrote the Japanese sentences guessin what you wanted to mean, if it isn't so, pkease let me know.