Leaving aside the issues of nuance and personal preference, all three options are formally correct. In particular "engineers forum" and "translators workplace" are formally correct.

The word "forum" is your main noun. You can modify it with a possessive:

- the engineer's forum (the singular "engineer" can be understood to mean the generic engineer, not one particular engineer); or

- the engineers' forum (the forum being attributed to engineers at large).

Or you can modify the noun "forum" with a noun-modifier. Noun-modifiers are very commonly used in English:

health insurance, tax evasion, biology professor, computer virus, birth control, diet pills, etc.

The general rule, typically taught with unwarranted rigidity to grade-school students, is that noun-modifiers are usually used in the singular form, even if the reference is clearly to a plurality:

a four-star general, a five-star hotel, a seven-course dinner, etc.

However, there are noted examples:

arms control, sales division, weapons violations charges, etc.

These examples are from the following page:

http://www.link.cs.cmu.edu/link/dict/section-AN.html

As I said, I do not wish to cast a vote on a "preferred" choice or to address the specific nuances of each choice, but to specifically address the formal correctness of these choices, and in particular to correct the notion that "Translators Workplace" is incorrect.