Although the easy availability of wearable, radio-linked omnitool computers has allowed the development of convenient earpiece translator units, translation software itself often lags behind due to Council restrictions on artificial intelligence technology. Some high-quality diplomatic translators (not usually available without a government contract) provide fast and relatively accurate translation, even extending to secondary language characteristics such as inflection and emphasis, but most publicly-available software packs are limited to five or six compatible languages and frequently introduce grammatical or semantic errors into the translation. Visual text-recognition in translation is often difficult to use and can easily be confounded by partially-obscured text or illegible handwriting. Additionally, no method has yet been devised to suppress untranslated audio or text, resulting in a characteristic 'doubling' effect. For these reasons, the most common method of communicating with aliens is simply to learn a common trade language; soldiers, sec officers, aid workers, and other groups that must frequently navigate alien environments often retain dedicated organic interpreters in addition to in-helmet translation software. This process is to some degree complicated by the fact that not all species are physically capable of pronouncing the same phonemes, but this rarely interferes with comprehension and standardized approximation tables exist.
The primary official language in Citadel Space is Standard Thessian, a planned language constructed from several major asari languages shortly after the founding of the Citadel Council. As Standard Thessian is mechanically extremely simple (far more so than the still-extant asari languages that were used in its creation) and contains only phonemes pronounceable by all species, a pidgin variant has achieved widespread use in the Terminus Systems despite the lack of any central authority requiring its adoption. Speakers with no prior exposure are usually capable of achieving at least basic command of the language within a matter of days. Citizens of all Council member governments are required to learn it as part of their primary education, although individual species may retain additional official languages. Other languages are occasionally used in specific fields- for instance, most academic and technical papers are written in Yan-Tek, a salarian language with structural similarities to formal logic.
All citizens of the Systems Alliance are instructed in English during primary schooling, with Spanish and Mandarin Chinese being recognized as secondary official languages.
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Author's Notes:
While Mass Effect at least admits that languages other than English exist and might rise in prominence in the future, it's such a common language now that I have a hard time believing that it won't be at least one of the official languages of the Systems Alliance- the canon Codex's implication of humans almost completely forgetting it in favor of Lojban or Swahili or even the currently ascendant Spanish or Chinese is certainly interesting, but I just very much doubt it could actually happen without some sort of organized deanglification effort.
The slight 'weakening' of translation technology from what the canon Codex implies (and many fanthors subsequently run to the hills with) is partially for realism and partially to make the story better. I'm not convinced that a complete, real-time universal translator is computationally infeasible at Mass Effect's tech level (although I do have to wonder about the mechanical workings of an earpiece that apparently mutes out untranslated speech but transfers all other sounds), but I do think such a device would require an artificially-intelligent understanding of semantics in order to work and it's a major point that such things are Not Allowed in Citadel Space. I also just think that having to still deal with different languages using Engrishy computer overdubbing and good-old-fashioned living interpreters better fits the less utopian, more grittily military tone of Palaven's Dogs overall, that not everyone being able to afford Star Trek quality communications fits well with PD's themes of some Citadel species being more equal than others, and that without language having been swept completely under the rug I am able to use who is speaking or writing what as another channel to convey meaning to the reader. Also, that was a really huge sentence.