I am currently doing freelance translation for mostly English-Malay projects for various international clients. Personal experience, I don't think my proefficiency for any language has deteriorate rather with more than 50++ projects/jobs, both languages are enhanced, especially for Malay.

It is quite impossible for your proefficiency in the language to decline if you say you are working on projects on a constant basis. (Unless all your projects are just based on translation of certificates etc, simple formal official languages).

I would suggest read more books, articles in English. So many books are available out there.

I am assuming you are hired by a 'shell' translation company then? In a sense, these companies do not have their own in-house translators but rather they sub the job to people like you and me? If that is the case, yes, the deadline is very tight because these companies hire you to the messy ground work and then ask another 'reliable' proof-reader to proof-read your work.

Are you translating articles? If yes, which field? I do tonnes of technical translations, mainly for mobile apps and websites. So I would say I am familiar with most of the technical terms and hence, my Malay vocab in the field of translation is extremely vast.

That explains the deteroriation in language. I did a stint for viral websites before. The pay was ridiculously low and the content, even written in English, is utter bullsh*t. Hence there is no surprise if you are doing something similar, your level of languages will deteroriate.