I have exactly the same problem. We are migrating to a different host and none of my customized strings are being shown in the new site. I have tried the following:
- Copied the **_local folder to the new moodledata/lang-folder
- Purged all caches
- Unchecked the option "Cache all language strings" (just in case)
- Checked that the web server has full access to the lang-folder
If I open the language pack for customization I can see all my customized strings, but if I press the save button it tells me there are no modified strings.
The old Moodle installation was version 3.1.1+ (build 20160714) and the new one is version 3.6.2 (build 20190114).
Is there a difference on how they handles customized language strings?
I think I found the reason why it didn't seems to use my customized strings. I realized that some customization actually had been implemented on the new site but not others. The reason is that the default language pack is different on the two different versions of Moodle. In the language pack for Moodle version 3.1.1+ the Swedish translation for the string "myhome" is "Mitt hem" but on version 3.6.2 it is instead "Mitt Moodle". I had not customized that string as I was satisfied with the old default translation.