Claws mail depends on enchant, which ships with Slackware. If your language is another than a variant of English, you need to install language dictionaries in Slackware. Using enchant, there are two ways. The first one I’ll describe below. The second is explained in enchant’s man file under DIRECTORIES IMPORTANT TO ENCHANT (something like: place dictionaries for specific back-ends in ~/.enchant/[name backend]).
The method I used is adding the appropriate dictionary to aspell, one of the back-ends that enchant uses. An archive with extra languages for aspell is here.
Download the tarball and create a Slackware package with src2pkg:
src2pkg -CWD [name tarball]
Install the newly created package as root. Done.
If enchant somehow does not use aspell, putting a file called
~/.enchant might help. In the file, you need to put the line: