On Mars and Mercury, many ancestral Human languages brought from Earth evolved into House Languages for various of the Human Houses. These languages are held closely within a House, and used for secure communication, as the US Army used Navajo Codetalkers to communicate securely without worry of Japanese eavesdropping. Other Human languages became more widespread - regional languages on Mars and more frequently on Mercury. As humans are far more heat tolerant than Martians, the more sunward areas of Mercury are almost exclusively populated by Humans.
On Venus, without much Martian meddling, these ancestral Earth languages evolved into small pockets of City-State specific language. Each city state has a different language, evolved from different Earth languages. There is also a Trader's Pidgin which is understood most everywhere, but it is not a real language, being very limited by design, and not very useful outside of trade.
Martians also have a bevy of House Languages, each one specific to a House. Regions have their own languages as well, for common communication. There are also several Lingua Francas. For example, Old High Martian is an ancient language preserved for diplomatic and ritual usage. There are a different set of Lingua Francas on Mars and Mercury, with only Old High Martian being used in common.
Venusian Kraat have many languages of their own, though they have no Lingua Franca. Kraat languages are impossible for Humans to use or learn, as parts of their speech are ultrasonic.
The Galileani have a variety of languages of their own, all descended from Callistan bases. As Galileani are a type of fruit bats, they do not have ultrasonic hearing, and neither echolocate or speak in ultrasonic chirps as Kraat do. Humans are able to learn and use Galileani languages with no problem, though sometimes concepts are hard to get across.
Optional Rule: Languages
This is a three part option.
I) Each skill rank in Language represents a new non-native language. These "basic" languages are leaned to a conversant level, but do not indicate anything like fluency. Normal conversations are possible, but accents are heavy, vocabulary is limited, and grammar is simplistic. Example: Paula has Language+2, and thus knows English and Old High Martian as basic level languages, in addition to her native Italian.
II) Each Specialization in a language indicates a language which is used perfectly fluently, to native-speaker standards. The specialized language must be one already known at a basic level, and the character also learns one new language at a basic level. Example: Paula gains another rank of Language, working on polishing her Old High Martian, and bringing it to a level of Fluency - Language (Old High Martian) +3. She also gains the new basic language of Demotic Ganymedian.
III) Instead of taking a Specialization, a character may instead opt for taking a Language Group at basic level. Characters may not later Specialize in a Language Group. Instead, they may only Specialize in one language of this Language Group - for example, Russian. When taking a Language Group instead of a Specialization, the character does not get an additional basic language, as happens when Specializing. Example: Paula gains another rank of Language. Instead of Specializing in English or Demotic Ganymedian, she chooses to take a Language Group - Slavic Languages - getting basic use of all Slavic languages - Russian, Czech, Polish, etc.