If the TYPO3 backend does not allow you to edit translated content elements, this may be due to inconsistencies in the MySQL database. In this case, it is worth taking a look at the "Log" module first. If it does include entries like the following, the database's collation is likely inconsistent:
Uncaught TYPO3 Exception: mb_strlen() expects parameter 1 to be string, null given
This could e.g. be a side effect of a website or database migration if the servers involved use different character encodings.
Further technical information about collations can be found on the MySQL blog, e.g.: MySQL 8.0 Collations: Migrating from older collations
A simple solution
Fortunately there is a simple solution to this problem, all from within the backend. Before you start: It goes without saying that you have a database backup handy, just in case…?
1) Install the TYPO3 extension "sfdbutf8"
2) Switch to the corresponding backend module and check the collation of your database entries. Red and green table rows indicate an inconsistent database collation.
3) Change the collation of the database tables by pressing the button at the top of the screen. Note that changes only the database table settings and not the data therein.
Now the editor for content elements should work as usual again. By the way, we would recommend removing the extension after the change is complete.
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P.S.: Nowadays most systems should be using "UTF-8" character encoding by default, but it wasn't always so... For MySQL we typically use either "utf8mb4" or "utf8_general_ci".