At the moment if you change the dictionary in one window it changes the dictionary on all windows as well as the global preference. It would be good that the global preference is used as the default language and then setting a different dictionary in one window does not change it for all open windows or future windows.

The locations where you change the language for that particular window could then even provide a checkbox [x] make this the global setting (modulo actual wording *g*).

A lot has changed in spell checking since 2006. The "language per window" has mostly been implemented, since the language is set based on a variety of factors, see bug 1073827, comment #33, individually per window. Note also that the e-mail composition in Thunderbird and SeaMonkey is now decoupled from the global preference, so there you have a "language per window" (see bug 967494 and bug 1163395). I am therefore taking the liberty to close this bug. Further enhancements are discussed in bug 1073840 (global setting, see comment #1) and bug 1203024.