Having just installed Linux Mint 15 and firing up LibreOffice Writer I noticed that my spelling mistakes were not being highlighted. I found that if I had the language set up for English (US) it would work but switching back to English (UK) meant errors were not picked up.
Looking in Tools – Options – Language settings – Languages I noticed User interface was set to English (USA) with no option to change it to English (UK).
I thought I’d check out the system languages: System Settings – Languages
I got a pop up: The language support is not installed completely, so installed the missing packages.
After updating I restarted LibreOffice and the spell checker worked correctly with English (UK).