Um, this is rather erroneous. First of all, when talking about the pre-historical "German" tribes you generally refer to them as Germanic, which is also the term you use for their language. Now the reason Latin is used for many scientific terms is because it was lingua franca amongs scientist until farily recently. This does not go back to the Roman period. There are naturally influences on English dating back to the Roman period but they are more common everyday words (leaving the legal terminology to the side), index (index finger) or exit for example. It is similar in German. Now Irish, Welsh, Gaelic etc. are all Celtic languages. The Celtic tribes settled on the Brittish Isles before the Roman conquest. There are naturally Celtic influences in English. The last time the Brittish isles were conquered would have been when William the Bastard (Willam the Conqueror) made good on his claim to the throne of England. William was a French duke, and was of Norse decent (the Norman settlement in Normandy gave the region its name). The Norman nobles spoke Old French and much of the language remain in English (the Norman noblity spoke French for a long time after the conquest). The people who William conquered England from would technically have been the English (Harald Godwindson). Harald had just managed to defeat Haral Hardrada the king of Norway, who also attempted to assert his claim to the throne. Why did the duke of Normandy and the king of Norway have claims to the English throne? They had it through king Cnut the Great, who was king of Denmark, Norway and England. Most of Englands history since the eight century had been a back and forth between the by that time native Anglo-Saxons (who were of Germanic and Norse decent) and the Norse, who invaded and settled on the Isles. There are a lot of Norse words in English, particularly the words dealing with boats or seafaring. Similarly to how many of the Roman words dealing with wagons are of Celtic origin.