There is no such thing as a foreign language to an Amberite; as all shadow is a reflection of Amber, so all spoken language is a reflection of Thari. Thus Amberites can understand any shadow tongue, and shadow dwellers can always understand an Amberite, all the time, even if the Amberite simultaneously speaks with shadow dwellers who speak mutually incomprehensible tongues. Note, however, that it is possible for an Amberite to make themselves incomprehensible to shadow dwellers by using the Pattern - in that case they actually have to speak a language comprehensible to the listener. This is done by Corwin when talking to Ganelon in front of others in Chapter 8 of 'The Guns of Avalon'.
As written languages are far more arbitrary in form than spoken ones, they are harder for Amberites to understand without any prior knowledge, but even so, with time and effort they can usually puzzle it out.
This take on languages in Amber is basically derived from my not wanting to have to bother with language difficulties (which could easily arise between player characters and NPCs, and even between PC and PC) in the course of a game.
The thought does occur that this could lead to a situation in which people's names are not heard as simply names alone, but are actually heard by Amberites as the meanings of the words which make up the name (e.g. 'Christopher' would be heard as literally 'Christ-Bearer' or 'Bearer of the Anointed One', and so on for other names). This would be not unlike the situation in the Everway game setting, where the names of people are meaningful words rather than just names. How significant this is in a given game should be to the GM, though...