Scanlation is generally not officially approved and operates in a grey area, so you don't need to ask... or rather should not bring it to the author's attention if you want to keep getting scanlations.
As for how to contact translators, you would have to go to the translation group's page or social media and contact them. Depending on what kind of work it is and what the group focuses on you should choose to approach someone who most likely has an interest in the material.
There are also some translators who do commissions, but that will cost you money.
