You can create your own "Language", call it "Mike-English" and there you can change any string of Limesurvey to the text you want. It isn't necessarily translating, more a changing of text in your case. But it works the same as described on this page.
poEdit is a tool that helps you to "translate" those strings, or better to change it. It is not that hard.
If you go into the core files and change them, this can cause trouble when upgrading to another version (or even in the current version), especially if you consider the page about translating as too advanced
Thus the tip with the translation files, which is a lot easier to handle when it comes to upgrading.