I always travel in spring should be accepted: it is a much more natural response than "in the spring"
Did DL refuse a translation without "the"?
There are many little regional variations like this in both English and Spanish. Sure, it is a valid alternative, but it isn't actually misleading and I don't imagine it is a priority for the authors. Three years ago (or so) it took several months (and undoubtedly hundreds of error reports) before DL changed the software to accept "springtime".
Indeed, little "errors" like this probably prompt more people to read the Discussions where they might actually learn something new to them, so I often wonder how many little errors are included deliberately.