The figure you added in the question explains everything!
You have the line
in your file-local variables of the file vocab.org.
And latex-mode is exactly what you get!
If you need these lines in your org-file you should add a line with only the special form-feed character ^L in it right after #+END_LATEX.
You can input the form-feed character ...
To everyone struggling like me to obtain an "easy to make highlight style", Make something like this:
'(("*" (bold :foreground "Orange" ))
("=" (:background "maroon" :foreground "white"))
("~" (:background "deep sky blue" :foreground "MidnightBlue"))
("+" (:strike-through t))...
In general, you can only add-to-list to an already defined variable. The variable org-emphasis-alist is not defined until org-mode is loaded. So you need to delay the add-to-list until after it is loaded: just move it to (say) the end of your init file (assuming that somewhere in your init file, org-mode is loaded).
HOWEVER that is not going to work in this ...
Org provides a variety of hooks to modify text on export. So, re-using your regular expression and org's italic emphasis, one way would be
(add-hook 'org-export-before-parsing-hook (lambda (backend)
(replace-regexp "^\\(.*:\\)[ ]*$" "/\\1/")))
Of course, then it will only appear in the exported document. So you may want to define a ...
As a comment to the answer by wing (but not enough reputation points to comment), org-emphasis-alist is a customizable variable so that it can be customized via the customization menu. It is found in the Org group under the subgroup Org Appearance. Modifications applied via the customization menu should be persistent over sessions.