I see variants on this question from time to time, like
How does one get special characters, like left and right quotes, in Inkscape's text?
How do I type Greek/Russian/Chinese/Tamil/... in Inkscape?
Here's the answer as best as I know. I hope people can fill in the gaps, and perhaps work on making this easier to do from Inkscape.
If you only rarely need to type the odd ‘déjà vu’ or degree sign, then the simplest way is with a gui character selection program like gucharmap (Accessories>Character Map in the Gnome menu, or Text>Unicode character map in the Debian menu system).
gucharmap wishlist item (which I haven't yet reported): It might be nice if gucharmap made it a bit easier to find commonly-sought characters (introducing hierarchy into the long list of categories, or adding one or more pseudo-blocks of commonly-sought characters that might span more than one of the named blocks). (In its favour, gucharmap does have a search function that can search in character names & descriptions.)
Also, it might be nice if gucharmap were to to show any Multi-key combinations defined for the character (see below).
If you type non-ASCII characters a little more often, then you might instead use Compose/Multi-key key sequences. The file /usr/X11R6/lib/X11/locale/en_US.UTF-8/Compose lists available key sequences. Many of these sequences start with <Multi_key>. Some keyboards have a Compose key labelled as such. Some X sessions are configured to use the right `Alt' key as Compose/Multi. In my case, I use a ~/.xmodmap file that makes my print-screen key act as a compose/multi key: the ~/.xmodmap file contains a line
keycode 0x6F = Multi_key Sys_Req
and I do xmodmap "$HOME"/.xmodmap in my ~/.xsession file.
However, note that some of the key sequences of interest (e.g. Multi_key o o for degree sign, or Multi_key < " for open curly double quote and similarly the corresponding > and ' versions for closing or single quotes) are available only when using the X input method rather than gtk's default input method -- see below.
For typing extended blocks of non-Roman script, you'll want to use an input method. The only way I know of to switch between input methods in a running inkscape session is to open the Text dialog box (Ctrl-Shift-T or in the Text menu) and go to its Text tab and right-click in the text entry area: gtk should give you its standard text-entry context menu including an Input Methods submenu.
One can change gtk's default input method (usable in Inkscape's standard in-document text entry) with the GTK_IM_MODULE environment variable, e.g. export GTK_IM_MODULE=xim (X input method) or export GTK_IM_MODULE=scim (Chinese, if you have the scim package and its gtk input method on your system).
It would be nice if Inkscape offered an Input Methods submenu in its context menu for text entry. Abiword has code to do this in abi/src/af/xap/unix/xap_UnixFrameImpl.cpp:XAP_UnixFrameImpl::_runModalContextMenu, but I haven't looked into that.
Should Inkscape do anything else to make it easier to enter non-ASCII characters? Perhaps have an item in the Text menu to launch gucharmap?
Should something like this message go into a short tutorial available in the Help menu?
pjrm.