I will do my best.
1. Backup your original names files, which includes the first_names.txt and names.txt files in your OOTP Baseball 15 database folder. This is usually located in C:\Users\YourName\Documents\Out of the Park Developments\OOTP Baseball 15\database
2. Then use a text editor like Notepad in Windows to open one of the two files.
3. To find a letter with a diacritical mark that you want to replace, scroll down your file until you get to the Hispanic names. These are the names that end with a number 1 at the end of their line in the file.
4. Highlight one of the letters with a diacritical mark that you want to replace, such as á, and right-click it to copy it.
5. Click on Edit from the Notepad top menu and choose 'Replace'. Right-click and paste the letter, such as á, into the 'Find what' box and then type 'a' into the 'Replace with' box.
6. Click the Replace All button. This will replace all instances of á in your file with the letter a.
7. Keep repeating this for all letters you want to replace, and you may also need to copy and paste capitalized versions of these same letters, like Á, and replace them with capitalized letters like 'A'.
8. To ensure that everything has been properly replaced, do some searches again for your letters to make sure that nothing comes up in the file.
9. Then save your file, open the other names files, and repeat the same process.
As an alternative, you can check out this thread:
Questdog's Names for OOTP 15. He has downloadable files with the diacritical marks removed, but make sure to read his thread to understand how his changes will affect your game. You should be able to download the file from OOTPMods.com as well.