There are two broad traditions of the role of language in theology.
The charismatic tradition holds that true learning is revelatory, it comes from personal contemplation, wells up from the emotions, or appears in flashes of inspiration from direct contact with the divine power. It can't be taught, though a master can hint by example or suggest things the student should meditate on. Since nothing important can be taught the language of instruction is not important. There may be texts, but the best use of them is to read them for general feel and throw them away; if it wasn't clear, you are better off with your own personal misinterpretation that trying to follow someone else's path anyway. Magicians working in these traditions need not be literate, and often have only their native language.
The scriptural tradition holds true learning comes from meticulous study of the holy scriptures and the commentaries of prior masters. It is deeply rooted in the written word, preferably in the original language, and assumes texts have layers of meaning that become clear only on careful analysis. Magicians working in these traditions must be literate, probably in more than one script and language. The languages of the scriptures and early commentators are naturally the most important - Hebrew, Greek koine, classical Latin, Syriac Aramaic, classical Arabic, Sanskrit, Pali, literary Tibetan.... Grimoires in these traditions are often ridiculously cryptic, changing languages in the middle of sentences, heavy with allusions and symbolic meanings, and sometimes written in actual codes. One can spend days teasing the meaning out of a few paragraphs.
The other important language issue is what do supernatural powers speak. In some settings spirits can converse in any tongue, but in others you must deal with them in a specific language. Not all powers are good at languages; in Babylon the gods were addressed in Sumerian for 2 millennia after it ceased to be used secularly, and there is a tradition that Satan once set out to learn Basque (to better tempt its speakers) but gave up when after 7 years he still could not manage "yes" and "no".
In Western traditions God may speak biblical Hebrew or classical Arabic, and many powers use the universal language from before the confusion of Babel. Christian devils are likely to negotiate contracts in church Latin, and hold you to the letter of them, so it is important to be quite fluent lest some of the subclauses not quite mean what you thought... It is also possible some powers have their own unique languages - Elvish or Celestial or Enochian.
The ability to speak with animals is a common mystical ability. Exactly how useful this is varies; realistic animals are not too intelligent and can't tell you much, but in many stories some animals are at least as smart as the average human. Sometimes animals can understand or speak the local human languages; but animals can have their own languages, the system from GURPS Bunnies and Burrows does well for those settings. It uses two sorts of animal languages: racial languages which are as expressive as any language and use the standard language rules, and common languages which are pidgins understood by a large group of animals and like all pidgins lack the vocabulary to discuss unusual or highly detailed topics. The exact taxa within which racial and common languages are shared are set by the GM. Bunnies and Burrows racial languages range from Lapine (Family Leporidae, rabbits and hares, but not Pika) to Reptiline (class Reptilia, all reptiles), while common languages range from Common Lagomorph (order Lagomorpha, lapines and pika) to Common Lower Form (listed as worms, snails and slugs - covers about half the Kingdom Animalia...).
A wizard mark is a personal symbol which acts an extension of the magician as long as it remains intact. It can be engraved, written in ink, tattooed on skin, even drawn in the dust though it's expensive for something so temporary. It need not be visible, colorless paints are a common method; a high tech wizard might try microlithography.
The magician always knows where his marks are relative to himself. If it matters he is simultaneously inside and outside any barrier separating one from him. He knows instantly when one is destroyed; the wave of searing pain which leaves him mentally stunned tips him off. Other things can be felt through the mark, as if it were part of his skin, ranges for his powers can be computed from the mark, and by entering a trance and making a Path of Visions roll can see and hear through it. Finally anything on which the mark is drawn gets no resistance against any of his powers.
A wizard mark can be drawn on a living creature, and some magicians mark their apprentices to spy on them or enchant them more easily. But the mark counts as a part of the magician's body, spells worked on it get the full bonus for having the magician present and participating. This is a drawback to marking apprentices, since a sufficiently determined apprentice could amputate the mark, recover his resistance, and use that bit of skin to attack his former master.
The art of drawing mystical symbols helpful in conducting magical procedures. For every two points of success margin with a preliminary Symbol Drawing roll, add 1 to the ritual magic success roll. The method of drawing the symbols depends on the tradition, they may be drawn with chalk, made with sprinkled powder (often grain or flour, or colored sand), etched in the floor, painted in human blood etc. Non-traditional methods may be used, but the bonus is reduced to 1 per 3 points of success margin. A roll is required before each ritual to obtain the bonus, though in the case of permanent symbols it is to reconsecrate them rather than to draw them. The Verver Drawing skill in GURPS Voodoo is Symbol Drawing [Voodoo]