[RPG] Are skills that an animal companion has ranks in considered class skills

Your animal companion, like any other creature, can attempt to use any skill that does not specifically forbid its use without training, and does so just as any other creature does. However, its ability to do so may be restricted by its lack of training (such as in the case of Knowledge skills), and/or its lack of ability to communicate (such as in the case of Diplomacy). As well, there may be circumstance bonuses or penalties (there's a section on those in the core books) that apply, and I'd consider it highly likely that your DM may choose to make house rules about skills such as Craft, Spellcraft, etc.

For all future reference, the above considerations apply to anything with hit dice - they can try to use any skill that they're not specifically forbidden from using by either training or a creature trait (such as Mindless).

Speaking, No. Understanding, Maybe.

The few Familiars that can automatically speak, do so "as a supernatural ability."

From PFSRD:

Familiar Basics

Skills: [...] Regardless of a familiar's total skill modifiers, some skills may remain beyond the familiar's ability to use.

and further down that page:

Intelligent Animals

Increasing an animal's Intelligence to 3 or higher means it is smart enough to understand a language. However, unless an awaken spell is used, the animal doesn't automatically and instantly learn a language, any more than a human child does. The animal must be taught a language, usually over the course of months, giving it the understanding of the meaning of words and sentences beyond its trained responses to commands like "attack" and "heel."

Even if the animal is taught to understand a language, it probably lacks the anatomy to actually speak (unless awaken is used). For example, dogs, elephants, and even gorillas lack the proper physiology to speak humanoid languages, though they can use their limited "vocabulary" of sounds to articulate concepts, especially if working with a person who learns what the sounds mean.

So, to expand my answer a little... No they can't speak it, unless the animal has the right anatomy to produce speech. They can be taught to understand languages, though.

(You could argue that the gaining of a point of Linguistics reflects the months of learning a language, so that being Awakened is not a requirement.)

Familiars automatically meet the Intelligence requirement:

Familiar Ability Descriptions

All familiars have special abilities (or impart abilities to their masters) depending on the master's combined level in classes that grant familiars, as shown on the table below. The abilities are cumulative.

Master Class Level Natural Armor Adj. Intelligence 1st–2nd +1 6 ...