I am using Libre Office Version: 4.4.3.2, Build ID: 40m0(Build:2), Locale: en_US.
My OS is Linux Mint 17.2 Cinnamon 64bit.
My Spelling Checker does not work in any Mode.
I have tried everything I know, including a Google Search for my problem, but no help.
Please help.
There are various causes for spell-checker malfunction. The first one is lack of correct language pack, but this must be put aside because you are en_US which is always installed by package managers.
The second one is misconfiguration. You must look in multiple locations.
The most general one is in Tools>Options, Language Settings>Languages. In principle, Western is sync’ed with your OS and should read Default - <your_language> '<your_country>. If this is not the, case fix it. Check there is a check mark with ABC at left of “Default”. This means the dictionary and other language tools are installed for this language and national variant.
You can also force another language from the menu (without “Default”) but make sure the check mark is here, otherwise spellcheck won’t work.
Next, check Default Style configuration (F11 to display the styles side-pane and right-click on it to Modify). Go to the Font tab. Check Language is set to English (USA). If not, push Standard button so that it reverts to Tools>Options setting. Eventually, choose another language, again making sure the check mark is present (no check mark mean the corresponding language pack has not been installed).
If problem still persists, you may have forced (knowingly or not) another language, such as None, with a command or toolbar button. Select the dubious sequence (or select all text) and Tools>Language>For xxx>Reset to Default Language (xxx = selection, paragraph or all text).
If this does not solve your problem, follow @gabix’s advice and edit (do not answer) your question to provide more details on what you did and what you expected.
Thank you ajlittoz. Today something changed? I didn’t do anything so I am confused. Your Advice: “check Default Style configuration (F11 to display the styles side-pane and right-click on it to Modify).” - {F11} Never worked before, but now it did!? I didn’t change anything, but now my spelling is working 
Thanks for your advice, which I will copy and save for future reference!
I use Liberation Serif. After pressing F11 and going to modify and changing the type of font to Liberation Serif, it started spell checking again. Under that option it originally was set to Cambria.
I had this same problem just happen to me on Windows. In my case, I had recently upgraded from a very old version of OpenOffice.
Apparently, they changed in one of the versions the location that extensions are saved in (On Windows, everything is under your \AppData\Roaming tree), but when you uninstall or upgrade OpenOffice, those settings are left behind and re-used when you install again. Basically, an error was occurring any time that I tried to install any extension, which is the real problem, because all the dictionaries are actually extensions I believe.
To fix, I uninstalled OpenOffice, then deleted the all the AppData files under \OpenOffice subtree), then reinstalled, and it set up a new tree and the dictionaries work again!