It depends on how the scale is disseminated and applied. Most of these tests can be used as is, but some tests are on a fee per use base. You will need to check how it is administered.
Do not use "did not say anything about copyright" as the benchmark. The authors might not be aware of it, but they can still own the right.
I am not sure what is the custom in your field but I would at least e-mail the authors to inform them. If anything having own's work adopted elsewhere is a great recognition. They may also give you advices on validating the translated scales. And of course if you go on to publish it, citation of their work will be expected.