French, from Ludovicus, the Latinised form of the Old German Chlodovic, "famous warrior".
Louis was Anglicised as Lewis. While both spellings are used in English-speaking communities, Louis is favoured in the US while Lewis is more common in the UK. Both spellings can be Anglicized forms of the Irish Gaelic names Laoiseach and Lughaidh [q.v.].
Several spelling variations occur in the 1841 Census of Aberdeenshire for Louis/Lewis [q.v.]. These include Lewas, Lewes, Lewies, Loues, Lues, Luis and Lowis. When searching old records, these spellings should be considered as possible variants. Most or all should be returned by using Soundex searching.
Louis appears as Ludovicus in old documents in Latin with appropriate Latin endings.
There are examples of at least two men called both Lewis and Ludovick on Parish records in Aberdeenshire. We therefore believe that Lewis/Louis and Ludovic/Ludovick should be considered to be synonyms, in Scotland at least.