Runes!

Runes are the letters in a set of related alphabets known as runic alphabets, which were used to write various Germanic languages before the adoption of the Latin alphabet and for specialised purposes thereafter. The Scandinavian variants are also known as futhark or fuþark (derived from their first six letters of the alphabet: F, U, Þ, A, R, and K); the Anglo-Saxon variant is futhorc or fuþorc (due to sound-changes undergone in Old English by the names of those six letters).
Runology is the study of the runic alphabets, runic inscriptions, runestones, and their history. Runology forms a specialised branch of Germanic linguistics. The earliest runic inscriptions date from around 150 AD.

So the German tribes were not the illiterates we thought.

We find runes alphabet on inscriptions.  Any other writing would have been on wood products, long since decomposed. I image folks carved the characters into wood ornaments. The technology was primitive in northern climes,there was no papyrus or anything obviously like it.

The rest of grammar was taught via poems and song and music. The system had fixed verse and stanza to a regular eight count. The system needed the rulers well annunciated poet class, intelligencia who would compose and refine legends before giving forth and maintaining the language of the realm.  Absent writing, there is a strong drift tendency as locals warp the grammar to fit local needs pretty quick, slang is always easy to use around the hood.