sorry, I missed the version number.
if a .host.resx or .portal-x.resx file is listed inside the language editor in the tree on the left, please stay away from editing it, always select the note w/o extension.
Q1: it depends on the HTML editor installed, how much it scrambles the text - there is a reason, why we always prefer CKEditor (will ship with DNN 7.4.0 and is planned to become default editor in DN 7.5.0). Download for free from http://dnnckeditor.codeplex.com
. You may as well use any other WYSIWYG editor to edit html keys and past the text into the text box in language editor.
Q2: files .resx are from core, .de-DE.resx or .fr-FR.resx etc. are from language packs (editable by selecting "System" in Admin languages). These files will be overwritten on upgrades of core, extension or language packs. .host.resx or .de-DE.host.resx are adoptions for all sites in your DNN. These files will be preserved on upgrades and (virtually) merged with content of the langauge packs. same for files .portal-0.resx or .de-DE.portal-1.resx , which affect only a single site inside your DNN.
files ending .host.host.resx or host.portal-0.resx are caused by a bug and may be safely deleted.
Q3. if you delete the files, you will loose your adoptions, yes.