Good thinking! I have a site where all the fields had to be translated. Not just theONLINE_STORE_THEME translations. Here I used your system with a different formula. I’m going to take your steps and put my formula down bellow for anyone would need it 
- Put both CSV exports in one Google Sheets file as separate sheets, and rename the old theme’s export Sheet2 and the new one Sheet1.
- On Sheet 1 in cell H2 (which should be the first of an empty column), paste this formula:
=IFERROR(INDEX(Sheet2!H:H, MATCH(1, (Sheet2!D:D = D7492) * (Sheet2!G:G = G7492), 0)), "")
- Then auto-fill all the way down. This formula is matching based on column G default content. Where it finds a match between column G in Sheet1 and column G in Sheet2 it copies over what it finds in column H in Sheet2 to column H in Sheet1.
- Export Sheet1 as a CSV.
- Ensuring that the ‘new’ theme is live, i.e. without the translations in it, import this CSV.
Thanks again!