The spellcheck dictionary shipped with Libre Office 3.5.0 marked as "English (USA)" is not English. It appears to have been replaced with "Aragonese".
Can't confirm this on my system. The US spellchecker doesn't work (that's another matter), but the entries in share/extensions/dict-en/en-US.dic are certainly (mostly) English!
Created attachment 57917 [details] Screenshot of spellcheck dialog somehow using the wrong dictionary to spellcheck english
I have also confirmed that the installed English dictionary in C:\Program Files (x86)\LibreOffice 3.5\share\extensions\dict-en appears to be correct. However, I still get bad spellcheck results for English which actually correspond to the spellcheck suggestions to Aragonese. Screenshot attached.
Also reported (and fixed) as 'Bug 47736 - SPELL: English spell check offers Spanish replacement options'. [Typo in 'dictionaries/an_ES/dictionaries.xcu'] IMHO a duplicate of bug 47736. *** This bug has been marked as a duplicate of bug 47736 ***