That is how I translated it ("The following field is required:") . I am not certain that it is 100% correct, though.

I interpreted

as the starting fragment of a sentence, that will be completed with

to make a full sentence, when the respective field is empty. But "The following field is required:" should also work, I believe.

That is currect - as stated in an earlier topic, IIRC. The code contains "payflowproGatewayErrorMsgJs + currField"

I've translated "The following field is required:" because in Galician "your" could be "o seu" or "a súa" depending on the noun which is next to it.

Toliño^{Fala aquí comigo}‎

It would also have been possible to move the translation of "your" out of the first message and into each of the others.

Yep! Did precisely that.

{{msg-mw|Payflowpro gateway-error-msg-js|notext=1}}