I cannot help but notice some 'r'-s seem to have randomly disappeared in both German and English. What is going on there?
Different reasons. 'they speak/they spoke' in Protogermanic was something like 'sprekanþi/spurkun'. Note the different position of the 'r'. Our forebears made the word more regular with the Germans going for 'spr-' and '-k-' whilst the Angles going for 'sp-' but also '-k-'. 'world' and 'Welt' are both contactions of 'weraldiz' ('men-age' or 'generation'). Contractions are always fairly irregular. The Germans and the English contracted the word slightly differently.