OP has asked:
>In 'the German alphabet', is 'German' to be interpreted as.      
 i) the proper adjective 'German'     
ii) the proper noun 'German' (the language, not, e.g. German citizen), acting as attributive noun modifying 'alphabet' or        
iii) one or the other; both interpretations are grammatical, maybe different semantics ?

In "the German alphabet," "German" specifically refers to the language, rather than the country.   
Let's look at the meaning of **"proper adjective"** and **"Attributive nouns"**
1. **Proper Adjectives:** A proper adjective is an adjective derived from a proper noun. Most language names are proper adjectives. Examples: German, French, Italian, Arabic, Chinese.
2. **Attributive Nouns:**:An attributive noun is a noun that modifies another noun. Considering language and their alphabets, examples include:

- The German alphabet
- The French alphabet
- The Italian alphabet
- The Arabic alphabet
- The Chinese alphabet

***"In these cases, the language name ("German", "French", etc.) can be interpreted as both a 'proper adjective" and an "attributive noun"***.