does one write "german language" with lowercase or uppercase g ?

The correct way to write "German language" is with a lowercase "g." The word "German" should be capitalized when it is used as an adjective to describe something that is from or related to Germany, as in "German food" or "German culture." However, when it is used to refer to the language spoken in Germany, it should be written with a lowercase "g."

But German is a proper noun so it should be capital even for language, or?

You are correct that "German" is a proper noun, which means it should be capitalized when it is used as a name for a specific language, such as "the German language" or "German as a foreign language." In this context, "German" is functioning as an adjective, and it is being used to describe the language. Therefore, it should be capitalized. However, when the word "German" is used as a common noun to refer to the people, culture, or things associated with Germany, such as "German cuisine" or "German cars," it should be lowercase, since it is not functioning as a proper noun in those cases. So, to summarize, "German language" should be written with a capital "G" when it is used as a name for the language, but "German" should be lowercase when it is used as an adjective to describe something related to Germany.