Extending plugin, WordPress theme not acknowledging

Hello,

I am currently running a login plugin located in the /wp-content/plugins/ folder. My website is dual-lingual and because the plugin is only in english, I have wrote an “if statement” script to change the text to french based on if the user clicks the french language on the website.

I would just like to extend this plugin and because of updates, I wanted to put this altered .php file into my themes folder so that my theme will add this to its functions but WordPress is not acknowledging this new change. Is there something I am doing wrong? Please give me insight, thanks!