He's slower than me but he's alsoand less productive than I am.

Your meaning here is a bit confusing. "But" would imply some sort of contrast between the first and second clauses of your sentence. However in this case, the fact that he is slower would lead the reader to already expect that he is less productive. So "and" makes more sense.

Whenever I slow down a little bit, I mean whenever I translate less than what I usually translate, the superiors immediately realize it and gather all of us, the whole team, for a meeting to bash us over that.

"gather us" is not grammatically incorrect, but the typical usage for a native speaker in this context would be "gather all of us".
Also, I didn't mark this as a correction, but the style of this sentence with all the different clauses separated by commas is something you would usually encounter in casual spoken conversation rather than in formal writing.

Then, as if ithat wasn't enough, he came up to my desk and threatened me with closing the Tamazight (Berber) version of the website I worked for.

Your original is grammatically correct. However typical native English usage would be to use "as if that wasn't enough" where "that" refers to some circumstances which you have already described or "as if it wasn't enough that ..." when you are about to describe the initial upsetting circumstances.

When my co-worker came, I scolded him in my turn, telling him that it's all because of everyone, not just me...

I'm the one who translates the biggest number of stories, and seeing that,. None of the others aren't as productive as I ame.

So yesterday was my opportunity to tell thehim: either you produce the same number of translations as meI do, or I'm not fillcovering up for anyonyou anymore.

Based on the earlier parts of the story, you are speaking to a single, male co-worker, right? In that case, use "him". "Them" would imply you are speaking to multiple people or a single person of indeterminate gender. "produce the same number of translations as I do" is grammatically correct, although you might hear "produce the same number of translations as me" in casual speech. "Covering for you" (meaning either to make up for someone else's poor work by working harder yourself or to hide from management the fact that the co-worker is the one who is failing to do his job) is the common idiom I'm familiar with. If there is a dialect of English which uses "filling up for someone" to mean the same thing, it's not one that I've encountered.

They all promised they'd be much more productive than they arehave been, but only time can tell.

He's slower than me but he's alsoso is less productive.

Whenever I slow down a little bit, I mean whenever I translate less than what I usually translate, the superiors immediately realize it and gather us, the whole team, for a meeting to bash us over that.

When my co-worker came, I scolded him in my turn, telling him that it's all because of everyone's fault, not just mine...

If the management wants to close down the website, fine.

I speak 6 different languages and I can take care of myself anyways.

He remained silent from the beginning to the end.

They all promised they'd be much more productive than they arehave been, but only time canwill tell.