According to Wikipedia, the oldest writing in a Germanic language -Alemanic- is from the 6.century, the so called Elder Futhark inscriptions. They only survive in pieces, though. The first full surviving texts are from the 9th century (the Hildebrandslied, the Muspilli and the Merseburg Incantations). The first glossary already exists since the 8th century. As far as I understand the songs are in Saxon, which would put them into the Northeastern part of Germany.

Also look for Althochdeutsch in: http://en.wikipedia.org/wiki/History_of_German http://de.wikipedia.org/wiki/Deutsche_Sprachgeschichte The German Wikipedia page has example texts (e.g. the Hildebrandslied), if you're curious what the language looked like back then. Hard to understand even for native speakers.