[ElMiko]

I'm very familiar with the romance languages, having studied 3 of them. As for the rest, the internet proved conveniently educational. The only one for which I'm afraid I might be missing an article or two is Greek (It's Greek to me! Ha. Ha. Oh geez, I'm too much). I also omitted the genitive case for articles since "title sort" doesn't modify it in English (Of Mice And Men).

Additionally, I'm not hating on Slavic or Asian languages by omitting them. In the course of my research, I found that many of the Slavic languages (and Arabic!) express articles as suffixes (or simply don't have them). As for the Asian languages, my own study of Chinese and my passing exposure to Japanese (neither of which have articles of any kind) leads me to believe the same is likely true of other East Asian languages. That said, if anyone else speaks Korean, Thai, Vietnamese, Khmer, or any of the Austronesian languages, please contribute! As for South Asian languages, I plead total ignorance, but I figure Kovid would be able to provide some input there.

Now, I realize I've only provided half (the easiest half, no less) of what you asked for. Unfortunately, regex is one language that I don't understand at all. Still, this list (and any others that forum members might contribute!) could serve as a reference in the even that someday someone gets an irresistible urge to create a bunch of regular expressions for relocating definite and indefinite articles in a title sort.

Spoiler:

Spanish
El
La
Lo
Los
Las
Un
Una
Unos
Unas

French
Le
La
L'
Les
Un
Une
Des
De
Du
D’

Italian
Lo
Il
L'
La
Gli
I
Le

Portuguese
A [This can also mean "To" as in "To Whom It May Concern"]
O
Os
As
Um
Uns
Uma
Umas

Romanian
Un
O
Nişte [note the non-standard letter]

German
Der
Die
Das
Des
Den
Dem
Ein
Eine
Einen
Einem
Eines

Dutch
De
Het
Een

Swedish
En
Ett
Det
Den
De

Turkish
Bir

Afrikaans
'n [Never capitalized]
Die

Greek
O
I
To
Ta
Tus
Tis
'Enas
'Mia
'Ena
'Enan

Hungarian
A
Az
Egy