The spell-check feature doesn't work by default in Pidgin on FreeBSD, but it's very easy to get working. Before making any changes to your system, you should double check that the feature is enabled in Pidgin. From the Tools menu, select Preferences, go to Conversations, and ensure that Highlight misspelled words is checked. If you're lucky, that could be all you need to do. If it still doesn't work after enabling that setting, you can try installing a dictionary such as en-hunspell:
pkg install en-hunspell
Of course, use a different 2-letter language abbreviation if English is not the language you want to check. Then fully exit and restart Pidgin for it to load the dictionary. That should be enough to resolve the issue for most people.
If you prefer not to install an extra package, you can install a dictionary similar to how you would manually install spell-check support on Windows. Simply download OpenOffice's U.S. English dictionary and save it to /tmp (or another convenient location). Then extract the required files as shown below:
unzip /tmp/en_us.oxt "en_US.*" -d /usr/local/share/enchant/myspell
Again, you'll need to fully exit and restart Pidgin for it to load the dictionary. The example above installs the U.S./American English dictionary, but you can search for a dictionary for your preferred language if you have another preference.