If you add a word in one language pair, then select a new language pair and try to add the same word, it brings up the information from the first language pair.
The way Readlang is designed is to permit learning of multiple different languages, allowing you to switch between them and keeping your texts and words very separate between each language. But when it comes to your first language, it assumes this will always remain the same. If you do switch your first language, what you described can happen. Sorry about that, but this was an assumption I made very early on in designing Readlang and would be tricky to change now.