I think "flashed" indicates it is sudden.

The french tense imparfait "brillait" means there is a duration, otherwise it would be "brilla".

was flashing ?

You're right. Things usually just flash once, but they can twinkle for a while... right?

Thanks!

Yes it can.
What the French sentence means is that his eye shined for a long time, like several hours or a week or a year...like if he was very happy...

I understand completely. I could change it to a past progressive / past continuous construction, but I think it'd mean roughly the same thing in English.

twinkled / was twinkling

I can change it, if you'd like.

That's OK, I trust your feeling.

But that's not quite what the French says.

My twopence :

Was glowing/ glittering...

Some tentative context :

Since last month when Mary told him that she might consider dating him someday...

Changing to "was shining"

To shine seems to be an acceptable translation... And it does sound better. Thank you for your help.