The left hand or right hand twist has ti do with the usage of such linen in a sewing machine/stitcher. For hand sewing it has no effect so use what you can find - been using them interchangeably for the past 50 years of hand sewing leather.
For light weight leather such as used for pouches a 3 or 4 strand weight/gauge thread is just fine. Use a #2 or #1 egg eye harness needle.
Linen threads such as Barbour are usually sized in both gauge and number of strands. For instance an 18/3 thread consists of three strands or cords of 18 gauge thickness. An 18/25 is 3 cords - making the 18/25 thread the same number of cords but thinner overall thickness since 25 gauge is thinner than 18 gauge (similar to wire).