In reply to Language and writing system:
These are my current thoughts on this topic:
No particular script is linked in any limiting way to a particular language. This is clearly demonstrated by the ease with which scripts are adapted to multiple languages. Indeed, there are relatively few scripts anywhere in the world that are used to write only a single language: everywhere the tendency is for scripts to be adopted or adapted to multiple tongues.
Yet a writing system is a tool for capturing language, so connections do exist even though they are not limiting. Most of the time, I use the terms writing system and script as synonyms (generally preferring the former term, as script has several other meanings), but there is a more precise use of the term writing system that applies at the level of connection to particular languages or groups of languages. Aleme has already referred to the adaptation of the Arabic script to write Persian and Urdu. What is the nature of that adaptation vis à vis the system of the script? It is what I would characterise as an extension: the nature of the system, the mechanisms it uses to capture language, are essentially the same, only the set of signs is extended. Contrast this with the adaptation of the Arabic script to write Kurdish, though, in which the actual system is altered. To someone who can only read one or neither of the Arabic and Kurdish languages, pages of text in the two might look very similar indeed, but one writing system is an abjad and the other is an alphabet. One script: two writing systems.
I do think it is useful and important for type designers to understand how writing systems work as systems. One of the workshop I run periodically for the MA programme at Reading is focused on getting the students to think in these terms, beginning with a lecture using a diagrammatic approach to the structural representation of phonetic information in various systems.
Thinking about a writing system as a system means understanding how the script captures language, understanding it structurally and not just visually. How important is this? Well, visual understanding -- understanding the set of signs as graphic objects, what they look like individually and how they appear in combination -- can get you a very long way, and there are doubtless good type designs that have been achieved with little more than this. But structural understanding provides insights that affect decisions about readability: if you understand the kinds of information that the reader is looking for in the text and needs to get out of the system, then you can make better decisions about how to present that information within the glyph shapes and their interactions.
There are degrees to this understanding, and at some point the truly useful and the personally fascinating are probably distinguishable. Most people understand that the European alphabetic writing systems all capture both consonant and vowel sounds as in-line letter signs, even if they've never articulated the understanding in such terms. Many, but fewer, also understand that many of these writing systems also employ digraphs, trigraphs, etc. to capture sounds not represented by individual sounds. Many understand that secondary marks applied to the basic signs may represent distinct sounds, but fewer appreciate that these may be considered letters in the application a system to one language but as diacritics in another. Some of this understanding is doubtless important to the type designer, but is all of it? What about an understanding of Turkic vowel harmony rules as reflected in the structure of the Uyghur writing system (something that Tom Milo and I spent an evening examining a few years ago)?
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The great majority of linguists simply ignore writing as secondary (and mostly inaccurate) notation systems for spoken language. This is why phonetic transcription systems like the IPA alphabet exist: to make up for the shortcomings of natural orthographies in capturing speech, enabling the linguists to ignore writing. One of the few exceptions to this is Florian Coulmas, and his book Writing Systems: an introduction to their linguistic analysis is worth at least dipping into.