Mixing everything up you get:
% !TEX TS-program = lualatex
\documentclass{article}
\usepackage{polyglossia}
\usepackage{mfirstuc} % << fot capitalisewords command
\usepackage{luacode} % << for luacode environment
\setdefaultlanguage{english}
%% Lua code - Credits to Mico
\begin{luacode}
function textreplace ( buff )
buff = string.gsub ( buff, "key sentence one", "\\capitalisewords{%0}" )
buff = string.gsub ( buff, "key sentence two", "\\capitalisewords{%0}" )
buff = string.gsub ( buff, "key sentence three", "\\capitalisewords{%0}" )
return buff
end
\end{luacode}
\newcommand{\TextreplaceOn}{\directlua{luatexbase.add_to_callback( "process_input_buffer" , textreplace , "textreplace" )}}
\newcommand{\TextreplaceOff}{\directlua{luatexbase.remove_from_callback( "process_input_buffer" , "textreplace" )}}
\begin{document}
Writing your document normally with key sentence one, other key sentence two and maybe also a key sentence three and the output is:
\TextreplaceOn
Writing your document normally with key Sentence one, other key sentence two and maybe also a key dentence three and the output is
\end{document}
For each new key sentence add a new buff = string... line. But you have to compile with LuaLaTeX, obviously (I say but because there is few downsides on doing so).
Sources (in case you want to do further customizations):
https://tex.stackexchange.com/a/78217/114143
https://tex.stackexchange.com/a/165224/114143
https://tex.stackexchange.com/a/305248/114143
EDIT: Taking into account what Werner had said in the comments: since the Lua code is processed before TeX does its magic the key sentence when TeX encounters it is longer key sentence, it's \capitalise{key sentence}, therefore it can span multiple lines and still the compilation will occur normally. Furthermore a hyphenation package can be still be used (polyglossia since there's need for LuaLaTeX). Also, from the third source I've posted, the Lua code can be activeted and deactiveted. I've updated the code to show this effects and implemented the activation macro.
Here is the compiled example:

sed. – Werner Oct 11 '16 at 15:17