Translation:The Swedish forests are full of spruces.
in general it would be more normal in english to write "the forests are full of spruce" if what is meant is the vast quantity of spruce trees generally and "the forests are full of spruces" if one wants to call attention to how many different varieties of spruce trees there are in these forests. But I think both are technically okay in this context, just that "spruces" sounds a little strange.
"Svenska" is the definite/plural form of the adjective "Svensk"
Plural forms are used with plural nouns. Definite forms are used with definite nouns.
Since "Skogarna" is both plural and definite, it requires 'Svenska'.
In the USA we also refer to these as evergreens. I did not try it but I was curious if it is an accepted translation? Thanks.
Evergreen just refers to any tree that doesn't lose its leaves in winter and includes (but is not limited to) all conifers, whilst spruce is a specific species. If I remember correctly, the Swedish for evergreen is wintergrön.