A general rule when writing an English letter to a client is that the language must be simple, while other "official" contracts, statutes, and legal documents tend to contain structures that may be difficult for a lay person. let alone a student who is not a native speaker. It should be noted that the majority of UK lawyers rely on templates when writing contracts.
The English law is derived from a mixture of old French, Latin, English, and Norwegian (Viking). This is because different countries invaded the UK hundreds of years ago.
For 300 years, French was the official language of England while English was the wide language using by general population, French was used in court proceedings and Latin was used for the official documentation.
This is a sentence written in Legal English (a special language of lawyers):
"I return here with the stipulation to dismiss the above case; the same being duly executed by me."
This sentence includes a mixture of French, Latin and English words and is a common sentence in English law. Some lawyers still write like this in Legal English documents. But what is it simply expressed in general English?
"I enclose the document to dismiss the case which has been signed by me."
Although this is a significantly easier sentence, it does not so legal. This is perhaps another reason why lawyers in the British and the United States tend to use more legal terms - they need to prove their higher education.
If you ask a lawyer why he uses nouns instead of verbs, writes in passive or uses Latin terms, they will say that is to avoid uncertainty, but many people think this is meaningless because if you have good English ability, you can write a watertight document without the old terms.
Languages in Legal English occurred since the British invasion
The Plain English and Legal English UK campaign have been trying to encourage lawyers to "tidy up" the use of these rules. Personally, I think this happens because these phrases have been used for hundreds of years and have been ingrained in legal lexicon. It will be difficult to change the habits that have appeared for hundreds of years.