Which language did these tribes speak?
The invading tribes spoke similar languages (all of them shared the same language branch), which in Britain developed into what we call Old English. Old English didn’t sound or look like English today. Native English speakers now would have great difficulty understanding Old English. Nevertheless, around half of the most commonly used words in PdE (Present-Day English) have Old English roots. The words be, strong or water, for instance, derive from Old English.
What did the Old English sound like?
Old English was closer to modern Dutch or German. All letters were pronounced. There are many sounds which don’t turn up in PdE like the /x/ or /ç/ which appear after a back or front vowel (when there’s an <h>). Actually, the word “right” was not pronounced as /rait/, instead, it was /riçt/.
If you hear it spoken or read aloud, you may be able to get an idea of what you’re hearing, but in general, you would have to study it like another foreign understand it. Let’s see an example from the Beowulf:
Old English (from Beowulf):
Hwæt! Wē Gār‐Dena in geār‐dagum
þēod‐cyninga þrym gefrūnon,
hū þā æðelingas ellen fremedon.
Oft Scyld Scēfing sceaðena þrēatum,
Old English Reviews http://oldenglish.umwblogs.org/audio-reviews/