Do this at your own risk.
In OFP's BIN subdirectory, there is a file named STRINGTABLE.CSV. You can easily open and edit this file using Microsoft Excel. Scroll down to around row 2070 and you should see the quotations table around there.
Each table entry consists of two rows. The first row of a quotation table entry, identified by the string "STR_QUOTE_n" in column A (where "n" is the sequential number of the quote/author pair of rows within the table), contains the actual text of the quotation in English in column B. Columns C, D, E, F and G contain translations of the quotation in French, Italian, Spanish, German and Czech respectively.
The second row of a quotation table entry, identified by the string "STR_AUTHOR_n" in column A (where "n" is the sequential number of the quote/author pair of rows within the table), contains the author or the source of the quotation in the previous row. The name placed in columns B through G is usually the same for all languages.
The last two rows in the table must always be identified by the strings "STR_QUOTE_LAST" and "STR_AUTHOR_LAST" respectively in column A.
IMPROTANT: The STRINGTABLE.CSV file is potentially subject to checksum verification and replacement when installing OFP upgrades and patches. So make a backup copy of both the original STRINGTABLE.CSV file and your modified version, just for safekeeping.