Final Fantasy X is the first and only Final Fantasy title to possess its own fictional "language". Although in actuality, the Al Bhed language is more likely an example of cryptography than language, it is treated as a natural language unique to the Al Bhed within the world of Spira, with its own unique written script and spoken sounds.
The Al Bhed language is based on a similar set of rules across different localizations of the game. It shares the same syntax and grammar as the Spiran language with special vocabularies and names kept in the Spiran language without encryption (such as the words fiend, machina, Rikku etc.) However, due to changes in the Spiran language depending on localization, the Al Bhed language is slightly different across the Japanese and English versions of the game.
In the English version of the game, the rules that apply to the Al Bhed language are based on English syntax and grammar (as English is the Spiran language in this case), and the encryption system is based on the English alphabet, where certain letters are replaced with others in the Al Bhed language. In the Japanese version, the Al Bhed language uses the same syntax and grammar as the Japanese language (since Japanese is the Spiran language in this scenario), however, the encryption system is based on Japanese kana (syllabic alphabet), and whenever someone speaks Al Bhed on screen, the subtitle is written fully in katakana (which is used for emphasis, foreign/loan words etc.), as opposed to a mix of the three scripts (hiragana, katakana and kanji). Due to the different alphabet systems between English and Japanese, spoken Al Bhed in the English version of the game sounds vastly different from spoken English (it sounded similar to German on first impression, and at times, it sounded like the fictional Sims language to me), whereas, the Al Bhed language doesn't sound very different from spoken Japanese to my ears in the Japanese version of the game. (You can compare the two versions here (English) and here (Japanese).
Within the game, although Tidus can become fluent in the Al Bhed language by finding all the Al Bhed Primers, very few people outside of the Al Bhed race can speak and understand the language fluently, which is especially noticeable within the span of Final Fantasy X. In FFX, the Al Bhed were the outcasts of Spira, and were obviously loathed, prompting one of Rikku's first questions to Tidus asking if he was an Al Bhed hater. This segregation was made more noticeable through the use of spoken languages within Spira. All races on Spira can speak the Spiran language to varying degrees of fluency. For example, the Ronso is a feline-like warrior tribe that tend to speak in very short and simple sentences in Spiran, and the Hypello, an amphibian race, tends to speak with adding extra nonsensical syllables to words, such as “yesh” (yes), “imposhibibble” (impossible) etc. but can still be made out to be Spiran that they are speaking. In the case of the Al Bhed, although the majority does not seem to speak or understand Spiran, as Tidus experienced when he first travelled to Spira, a handful of people are completely fluent in the language of the masses, such as Rin, Cid and Rikku.
After the fall of Yevon by the end of FFX, Spira's attitude towards machina and the Al Bhed race began to change. By the start of the events in Final Fantasy X-2, many Spiran has taken interest in Al Bhed culture and has begun to learn the language, some can even speak a little of the Al Bhed language, such as O’aka XXIII, the traveling merchant.