I decided to finalize the app to make it work, in case someone might need it right now.
I didn't make tooltips, I didn't save settings to json, I didn't do file upload via button and a couple other things, but I'm lazy. The main thing is that the basic functions work.
I recorded a video to make it clear how to use it.
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You may only need one function - "filtering strings", by the desired pattern and immediately find strings in Japanese only.
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You will see 2 drop down lists at the very bottom. You can select 1 or 2 languages there. The video shows how it works.
In the lower left corner is a box for lines to ignore. The default setting is "DO NOT EDIT". Rows with this content will not be displayed in the result window. You can also add other lines to the same window by pressing ENTER and paste your line on a new line. I probably should have made this window bigger.....
I made this app only for myself, and the string filtering function only for this game, so it's just a kind of prototype.
The application is made through PyQt5. I compiled the script into an .exe file so that it can be used by anyone who doesn't have Python, and also so that you don't have to install all the libraries needed for the application to work.
To tell you the truth, I don't know if this will work on Linux and MacOs, or if you need to compile your own executable for other OSes too.