Joking aside, the cases in German are not that difficult, since most of the time the nouns themselves don't change, only the articles or other determiners. In the Slavic languages, however, the noun itself changes its ending and on top of that, there are more cases. Same for Finnish, Hungarian or many other languages.
When referring to most languages, you write it (using this example) as "La Germana" (short for "The German Language") in esperanto. However, you write it simply as "German" in English. One exception us when referring to esperanto, which is just referred to as "esperanto" because it is not the language of any specific nation or group of people. This is a common problem for beginners, which I still am, but since you received four unhelpful downvotes and no useful response, I wanted make sure you got the answer.
Just German. If you are using "German" as a noun you don't use an article, because English doesn't use articles with proper names. (If you are using it as an adjective, then it depends on what is being modified. Thus: "The German boy sang a song in German to German Heinrich." "The German boy"; German is an adjective, boy is a noun, boy gets a "the".
"a song"; song is a noun, gets indefinite article "a"
"in German"; German is a proper name and doesn't get an article.
"German Heinrich," German is an adjective, Heinrich is a proper name, so it doesn't get an article.