Hi there,
We are also experiencing that same issue with a lot of "Missing translation" messages when adding H5P content to the content bank.
I've tried reinstalling all the other langpacks and clearing the caches but that didn't solve the problem. In our case this happens even while using the English language pack (note that some of the texts - Title, Panels - are well shown):
Looking at the logs I've seen this error message that seems to be related:
Default exception handler: Invalid content ID Debug:
Error code: invalidcontentid
* line 498 of /lib/setuplib.php: moodle_exception thrown
* line 67 of /contentbank/contenttype/h5p/classes/form/editor.php: call to print_error()
* line 214 of /lib/formslib.php: call to contenttype_h5p\form\editor->definition()
* line 59 of /contentbank/classes/form/edit_content.php: call to moodleform->__construct()
* line 95 of /contentbank/edit.php: call to core_contentbank\form\edit_content->__construct()
We are using Moodle 3.9.1, if you need more data tell me what could be meaningful for you.
Thanks!