Tłumaczenie - Angielski-Łacina - And Shepherds we shall be For thee, my Lord,...

"So we shall flow a river forth to Thee"

It doesn't mean they will boat up or down a river, but they will *make* a river flow [out of them] towards God ("forth to Thee"...

"And teeming with souls shall it ever be"

... and this river (faith? prayers? hymns?) will be ever (forever) full of souls (like "teeming with fishes".

Are you sure? we shall flow a river = we make a river flow? hm...

At least as sure as someone that is not a native English speaker. The Webster's gives, for "flow" in transitive use :
1 a: to cause to flow
b: to cover with water: FLOOD
2: to discharge in a flow
I think the only meaning that fits here is 1 a.

Confer "He grows flowers" = "He makes flower grow"
Easy way to express "factitivity" with intransitive verbs.

He makes flowerS grow...

Ok. I believe You're right. So I've made quite big shift in my translation according to your remarks.Check it, please. :-)

macte uirtute

Per Te id assecuta sum :-) Gratias TIbi ago!