How do I install a new dictionary for Komodo's spell-checker, such as British English or German.
Komodo uses Mozilla's dictionaries. Each dictionary consists of two files, ln-LC.aff and ln-LC.dic, where "ln" is the language's two-letter ISO language code, and "LC" denotes its two-letter locale code. Komodo ships with the files en-US.aff and en-US.dic in the <komodo-install-dir>/lib/mozilla/dictionaries.
You can either copy existing Mozilla dictionaries from other programs, such as the <thunderbird-install-dir>/dictionaries directory, or find the dictionary you need at http://www.mozilla.com/en-US/thunderbird/dictionaries.html
Download the .xpi into a temporary directory, unzip it, and copy the .aff and .dic files to <komodo-install-dir>/lib/mozilla/dictionaries
Komodo should offer the new dictionaries in the Check Spelling dialog's Language drop-down bar. There is no preference to change dictionaries -- Komodo assumes that the most recent language is your preferred one, and remembers it.
One thing to keep in mind is that these dictionaries install into the program's installed area, not the profile area. This is a known bug with all Mozilla-based programs, and work is in progress to improve this on an overall basis -- the new spell-checkers will reportedly be faster, support more natural languages, and will be installed into the profile area. For now, when you migrate from version 4.2 of Komodo to a newer version, you'll have to manually copy the dictionaries over (unless the Komodo installer does that for you at that point).