So, these two skills exist. Both are Intelligence based with Int and General Aptitudes. One is hilariously complicated, gets a whole column all to itself and allows an Acolyte to interpret and learn new languages, with a whole descriptive sidebar and examples box and everything.

The other is just: "Used to learn or decipher new languages, both spoken and written, and even create new ones in some cases."

To me, they sound like the same thing. Am I missing something that differentiates them, or are they the same and it's just up to you if you prefer the basic skill or the specialisation?

In my understanding, the skill Linguistics represents the languages the character can speak etc., such as low gothic.

The skill trade (Linguist) covers also the technical training to work with languages.

For example: I as a common 40k citizen can speak and write low gothic. But I could not translate some old texts found on Terra. I might understand a few words which look similar. While a trained linguist knows how to build up a scheme or build a basis to translate the whole text.

In games I handle it this way: Charactes who have the skill linguistics (specific language) can speak, write in it. If they want to translate a related language they get a malus, while a character with the trade (linguist) gets none...or a lower one...he would be a trained translator after all.

As far as I know starting characters can speak and write low gothic.

This is my personal interpretation:

Linguistics represents accumulated knowledge in the field of linguistics.

Trade (Linguist) represents your training as a linguist.

For example, if I learn German, Eldar, and how to interpret Imperial Codes, I've functionally learned some aspects of Linguistics.

If I practice in the field and know how to decipher unknown languages, or practice it as a profession to learn and teach languages or the interpretation of languages and their structure(s), I'm a Linguist, thus, Trade (Linguist).

They can potentially relate greatly to eachother, for someone that makes it a point to learn many languages and attain multiple ranks of both. But they can also be potentially unrelated, such as someone having no interest in linguistics, but that still learns a language or three.

I would represent this interrelation by giving someone with Trade (Linguist) cheaper Linguistics advances, if they practice as a Linguist, first. I'd let them roll Trade (Linguist) to interpret, for example, Eldar Runes or trying to make sense of the screaming of an Ork Slave. Then when they want to learn the proper Linguistics, they'd do that cheaper.

Or something like that.

I see linguistics as knowledge of a certain language, be it gothic, eldar or some artifical language invented by the inquisitor (acolyte cipher). The fact that this skill is called linguistics is a misuse of the term linguistics as linguistics is quite specifically not linked to a particular language but is a broader term. It should just have been "language (low gothic)" instead.

I see trade (linguistics) as the experience to work professionally with languages, like an interpreter or teacher. If the player knows multiple languages it might also encompass studies on comparing languages. So the trade skill does not convey any abilities in languages, but if the acolyte wants to pass as a teacher or do some professional grade translation this is needed.