In addition to https://tracker.moodle.org/browse/MDL-84763 which mentions the category name field being too short, the exact issue occurs on a field name.

Summarizing from the aforementioned issue, the field can only contain 400 characters maximum, which is too short when wanting to support a significant list of languages (if one can call 6 languages "significant")

Reproduction steps:

1. Make sure enough language packs are installed and multilang filtering is enabled
2. Open Course custom fields page by "Site administration" > "Courses" tab > "Course custom fields".
3. Navigate to one of the categories that are there and choose to add a random field to add (I used a "short text" field).
4. In the form that appears, enter a shortname and then, in the name field type "<span class="multilang" lang="en">A somewhat longer field name (EN)</span><span class="multilang" lang="de">A somewhat longer field name (DE)</span><span class="multilang" lang="fr">A somewhat longer field name (FR)</span><span class="multilang" lang="id">A somewhat longer field name (ID)</span><span class="multilang" lang="nl">A somewhat longer field name (NL)</span><span class="multilang" lang="tr">A somewhat longer field name (TR)</span>
   ", then try to save the changes.
5. Confirm you run into a "dmlwriteexception" which mentions that the data is too long (the actual message may vary depending on the database engine in use)