It’s true that TranslatePress can pick up a lot of text that doesn’t need a translation. Inserting text in the database even without a translation is a cost that we are aware of, but in most cases, there are no side effects. There is constant work being done on our side to better recognize what text is useful for translation and what’s not.
In addition, we are working on a tool to clean the database build up.
If you can identify the places where TP picks up dates and other untranslatable text you can make adjustments yourself by adding the data-no-translation attribute on the HTML. This will prevent adding them to the database as well as translating them.
About your case, changing the original content will invalidate the translation because their are string-based. Every string has a corresponding translation (if you create one). If you change the original string, basically, you create a new string that will need to be translated.
To facilitate translation retrieval we implemented the Translation Memory feature.
For example, if you have a string in your default language that is translated and you go in and modify that string, the translations for it won’t appear any more at the front-end level but will continue to exist in wp_trp_* tables from your database.
After, if you go to the translation interface and select that string, the Translation Memory feature will show you translation suggestions for that string (based on other strings that you have translated, even ones that don’t appear anymore in the front-end; so most likely, you will see the older translation as the first suggestion).
However, this translation memory feature is not 100% accurate and also could not return any suggestion sometimes.`
