The following is a spell checking method you can use for Ren'py games. You will want to do it after you complete a draft of your game or of a long portion of your game. I prefer to do it at the end of a milestone (I am doing it now after finishing Chapter 3 and I will do it each time after I finish a following chapter).
1) Open the Ren'py Launcher
2) Select your project
3) Select Extract Dialogue. Extract is as dialogue text only and strip text tags from the dialogue.
4) Click continue. When it's done, a dialogue.txt file will appear in the game's root directory.
5) Open this file with any word processor that has decent spell checking (I use Word 2016). On Word 2016 click "Review", then "Spelling and Grammar" and you can go through all of the errors one by one. If you see an error that shouldn't be there, you can change it in your original script. If you see something you use a lot that is mistaken as an error (i.e. "gonna" is flagged by Word) you can click "Add" to add it to your dictionary so it's no longer an error.
Side tip 1: you may wanna expand paragraph spacing or the font size/type to make the plaintext more readable in Word.
Side tip 2: unfortunately, dialogue.txt only extracts every .rpy file in what I believe is alphabetical order. So the order of dialogue in the dialogue.txt file may vary. If you have trouble finding where a line of dialogue that you need to fix in your original game folder's is, you can use ctrl+shift+f in your text editor and enter that incorrect line. In most text editors ctrl+shift+f will search for that line in any file in your current directory.
There's my tip. Dunno if there's better means out there but if you're too lazy to proofread (I know it can be so boring to do so much) then this can help cut down on it
But you should always do lots of proofreading and test playing, this tactic is only meant to cut down on that (not remove it).
