The only branch of Polabians who managed to repel the Germans were Lusatians or Sorbs, existing even now and speaking two Sorbian languages related to Polabian. Other Polabian tribes were assimilated, and their language is now completely extinct. The only sources for it are some little phrases recorded in the 17th and 18th centuries, and also placenames and some borrowings in German dialects which represent Slavic substratum.
Polabian was obviously similar to the older stages of Polish and Sorbian languages. But phonetics also shows some similarity to Baltic and Germanic languages, which makes us suppose that Polabian was quite archaic. E.g., Common Slavic *-ra- turned into -ra- / -ro- in all modern Slavic tongues, but in Baltic and Polabian it is -ar-. From the other side, Polabian has gone far in its morphological development, losing some noun and verbal endings, like the 3rd person singular verb -t. The infinitive was ending in -t which was not changing into -c' or -s' like in Polish, Sorbian and Czech.
Germanic influence was expressed not only in vocabulary, which contained about a half of German words, but also in phonetics, for some new vowels emerged in the language, similar to Germanic ü and ä.