Wondering, I'd like to change some language strings in-game, but I'm scared to corrupt my game. I mean, obviously not changing important text lists containing strings that aren't intended to be changed, but changing strings like "Bye." when visiting guests leave your lot to e.g. "Ta-ta." or something like that.

Obviously in case I did this, I'd have backups of everything.

(Please also excuse me if I placed this into the wrong section, I was a bit unsure if it fit here, but I guess I am asking for help?)

You can do that sort of thing as a default replacement - that is create a file to put in your Doiwnloads folder which will force the game to use your alternative text. No need to alter games files at all. How to do it is another matter. In principle it's quite strightforward - find the string you want to replace, find it's identification code, put that in a file with your alternative, save and put your file in Downloads. However, explaining how to do that precisely takes a good bit more time. Have a look in the tutorials section here to start to learn how. Look in the modding section. You will need SimPE.

Alright, I might be on the right track now. Thanks. Firstly I have to sort out some other issues with my game

It seems like you've already figured out how to find the appropriate strings. Once you've done that, right-click the resource in SimPE and hit Export. Don't change the filename. Then go File -> New in SimPE, right-click the resource area, hit Import and find the file you just exported. Make your changes, commit, save, and pop the file you just made in the Downloads folder.

This will work great for strings, BCONs etc. Be careful if you start messing with other types of resources doing this, as doing itwith specific object and CAS resources can enable you to delete maxis stuff from the buy catalogue or in CAS, which will corrupt your game. :P