I thank you for this useful information. I use Focuswriter and LibreOffice to write books on the RPi. I have found it much more comfortable to sit on my couch with a lap desk and illuminated keyboard at the end of the day when everyone else is asleep to work on my stuff.
This niggling difficulty has been giving my grief for some time now and it had such a simple solution I am almost embarassed that I had to Google for a solution. My copy-editor's life will no doubt improve drastically.
As to the post regarding children learning to spell and check their work without electronic assistance. Well, I suppose that is a valid argument. Having said this, may I respectfully point out that I am 60 years old now and learned to read and write without electronic aids of any kind. When I learned to type (Olyjmpia manual typewriter), I was introduced to a printed booklet entitled 20,000 Words, in addition to Webster's Dictionary of the English Language. These documents pretty much comprise a non-electronic form of myspell, the difference being that it takes longer to use them.
I am not an especially well-known author (yet!), but I hold copyrights on three books that can be found in the stock of most online booksellers. I am somewhat of an expert on the English language compared to most children. EVEN I MAKE MISTAKES with English... which is just about the most difficult language in the world to learn because it is comprised of so many foreign influences (Latin, French, German, Algonquin, to name a few). This makes the rules of spelling somewhat inconsistent. There is no solution but to try to remember as many words as possible, but even this is not perfect.
I postulate that if a child starts "typing" with myspell operating, thus having all his mistakes instantly underlined in red, he will not only learn to spell, but he will learn to spell more quickly than if he had to complete the document... say using a Royal Model 1910... and turn it in to his teacher for correction or laboriously go through the document checking each word against the dictionary or spelling guide. Tools are there to be used, not ignored. Spell-check is here to stay.
I also postulate that some typos, especially those like here/hear, one/won, etc., are invisible until after publication, even WITH spell check enabled. Anyone who went through any of my books (or anyone else's) with a proverbial fine-toothed comb would find an embarassing number of typos... and remember that my work and the work of most of my fellow authors is checked laboriously be people who get paid hard coin to do just that.
If one looks at the original solution, one will note that "apt-ger myspell-en-us" has a typo in it (the proper command is apt-get).
I will also note that this solution works for FocusWriter as well as LibreOffice Writer and probably Gedit, and others.
Again, thanks for this extremely useful post.
EDIT: Note that spell-check was NOT enabled when I made this post. Here at work where I have a more complete system installed, there is also an embarassing (misspelling intentional) number of typos. Hopefully when I return home and reboot Raspbian it will also work on Firefox. I am at once reminded of the "expert" in firearms handling who shot himself in the leg in front of a classroom filled with children. (He was, if memory serves, a DEA agent... check youtube). While bleeding all over the floor, he made a good point that mistakes are made by EVERYONE.