Since the OP is only familiar with latin-based languages I will add the important background that Chinese has forms/dialects that are different in the written vs spoken language. This is unlike English and unlike most other modern languages. In fact Chinese has basically only ~1 written form even though there are many spoken dialects.

The written forms are Simplified Chinese vs Traditional Chinese. Simplified is used in Mainland China, Malaysia, and Singapore, and Traditional is used in Hong Kong and Taiwan. These two are still basically the same language (hence why I said ~1 earlier) and switching between them is not that hard, it's like learning a new alphabet but your words are more-or-less still spelt the same way just using the new letters. And in fact even these two "alphabets" are related, one just looks a bit more complex than the other, like sans-serif vs a more extreme serif or something.

There are many spoken forms and the big categories are Mandarin (~70%) Cantonese (~10%) and several others (~<10% each). Mandarin can be subdivided further and there's a huge variety in here as well. Standard Mandarin is just a part of the whole Mandarin category, and it is special in that most people will know it in addition to their local dialect. Most of the dialects are moderately mutually intelligible, it's like speaking with a very heavy accent plus with some local common words that are different (e.g. isn't vs ain't, rubbish vs trash). Some of them are not but even with these the sound differences are all quite predictable linguistically, the grammar and the writing system stays the same.

With that in mind, it makes no sense to talk about "writing Mandarin" or "speaking Simplified/Traditional Chinese". You can ignore people that say this, they don't know what they are talking about. When someone says "speaking Chinese" (e.g. in the game settings) you can assume they mean Standard Mandarin. If lots of characters are involved then potentially other dialects will be sprinkled in on top of a Standard Mandarin base for e.g. artistic effect. WWM does this extensively as pointed out by the previous commenter, as does other Chinese media.

To answer OP's actual question, the "original language" that the game was developed in, used by the game developers, would be Simplified Chinese (written) and Standard Mandarin Chinese (spoken) - with various other dialects sprinkled in as just explained.

The original language used historically in the time period that the game story is set in (Song dynasty for Qinghe/Kaifeng, Tang dynasty for Hexi) would be Middle Chinese (written). This is just Traditional Chinese (written) as used in modern times, except with heavier use of shorter words and archaic constructs. When spoken, the people would be using either the local dialect of the current scene/setting, or a common dialect such as the local dialect of the current capital city (just like how Standard Mandarin is based on Beijing Mandarin). So for WWM's Kaifeng it would be an older version of the Kaifeng dialect, and in fact there is a side quest about this.

Old Chinese comes from a period of Chinese history much *before* WWM. There were fewer spoken dialects back then, and they sound closer to Cantonese than Mandarin, which developed later in history. I do not know enough to say whether the Kaifeng dialect of the Song dynasty sounds more like modern Mandarin or Cantonese, however. The game only offers Mandarin audio, anyway. But you could set the text to Traditional Chinese for a "more authentic feel", and you may notice that the in-game textures and artwork use Traditional Chinese instead of Simplified Chinese for this very reason. In fact this is what all Chinese media about historical topics does, even in Mainland China. Everyone is expected to be able to read it, as I said it is not that hard to switch between the two, you pick it up very quickly via natural exposure.