Changes have been made in Git (master):

commit bc4f11f1200a5bd0be091bb1545671971731b2fa
Author: Jan Schneider <jan@horde.org>
Date:   Fri May 27 19:46:38 2016 +0200

     [jan] Pre-select the current locale when building the 
spellchecker languages (Request #14244).

  framework/Core/lib/Horde/Core/Ajax/Imple/SpellChecker.php | 3 ++-
  framework/Core/package.xml                                | 2 ++
  2 files changed, 4 insertions(+), 1 deletion(-)

http://github.com/horde/horde/commit/bc4f11f1200a5bd0be091bb1545671971731b2fa

Unfortunately, this does not seem to be the case for me. The first 
language in the list is always the language selected when I pull down 
the list, regardless of what I had selected previously.

Also, I'm still seeing unreliable and inaccurate results until I 
manually select a language each time I compose a new message. Because 
it doesn't seem to guess the language correctly the first time nor 
remember my selection from the previous time, it's at best frustrating 
for our power-users and effectively broken for our less-experienced 
customers.

I think by default is always uses the language that was selected last. 
This makes sense and I don't see a reason to change this.

The spell check in the compose window will correctly guess en_US when 
checking the text "This is some speeling text", but it is not able to 
guess the correct language when checking the text "speeling". I 
suspect this is because it's doing some guessing and doesn't have 
enough input to guess correctly, which is reasonable but I'd prefer to 
be able to explicitly define a default spell check language. This 
wouldn't replace the built-in guessing, just inform the guessing if 
it's unsure. For reference, I'm already defining en_US as the default 
language in nls.local.php but it doesn't help. I'm submitting this as 
a request and not a bug because I think it's doing what it's supposed 
to do, but I think it could be improved.