If we would like to say 'save my life' in German, we cannot say 'sparen mein Leben'. But what is the reason?
Firstly, we can trace the origin of 'save' in English. In the early 13th century, 'save' typically meant 'to deliver (soul) from sin or its own consequence'. It came from Old French 'sauver' which was originated from Late Latin 'salvare'. It meant to make safe or secure.
Secondly, 'sparen' in German was imported in Middle English in 9th century. It becomes 'spare' in Modern English today. Though it means 'to save from embarrassment', it also means 'to refrain from employing'. Thus it means 'to conserve money' (Geld sparen).
We should say 'Retten mein Leben!' in German for 'Save my life!'.
How about 'retten'? Does it become 'rescue' in English? I guess no. 'rescue' came from 'rescourre' in Old French.