As long as its guaranteed to be sapwood free and slow grown, Baltic pine will be fine with a hardwood sill. Keeping on top of the maintenance is the key to keeping timber windows alive.
If they can't do the sapwood free bit, avoid softwood. Idigbo is one of the common hardwoods used for windows, Meranti is not as durable but is widely used too.
There are loads of tropical hardwoods with weird and forrin' sounding names. Most of these are fast growing without a distinguishable margin between the sapwood and heartwood, so you don't really know what you're getting.
Avoid Southern Yellow Pine for outdoor work, as it's about 80% sapwood and not durable, although it's hard and gives a good finish for interior joinery. They use it for crates and pallets in Americania.
Douglas Fir is a decent softwood, as it's moderately durable. Spruce is a no, no, Scots Pine (Baltic Pine) is good if it's slow grown and from big trees.
If I were you I'd approach your local small joinery shop and get a price from them.