Issue Summary

Performing any UPM update will flush & rebuild the caches (as expected); however, it will rebuild the caches using the locale of the admin that triggered the flush. Querying against the caches will then result in unexpected translation issues.

Steps to Reproduce

We can reproduce the problem using the Jira API. To simplify steps, we use a ScriptRunner script for this below;
1) Create to users admin_en and admin_fr and add them the jira-administrators group
2) Either during the user's first login or later using Profile/preferences screen, set admin_fr user's language to French
3) Login using admin_en, and create a new custom field named my.customfield
4) Add two translations to this custom field using translate context menu item;
4.1) for English, My Custom Field
4.2) for French, Mon Champ Personnalisé
 5. Run the following script using ScriptRunner console

import com.atlassian.jira.component.ComponentAccessor;
import com.atlassian.jira.issue.CustomFieldManager;
import com.atlassian.jira.issue.fields.CustomField;

CustomFieldManager customFieldManager = ComponentAccessor.getCustomFieldManager();

String CF_MYCUSTOMFILELD = "my.customfield"
String CF_MYCUSTOMFILELD_ENGLISH_TRANSLATION = "My Custom Field"
String CF_MYCUSTOMFILELD_FRENCH_TRANSLATION = "Mon Champ Personnalisé"

CustomField cf = null;

cf = customFieldManager.getCustomFieldObjectByName(CF_MYCUSTOMFILELD); 
log.warn("untranslated: " + cf)

cf = customFieldManager.getCustomFieldObjectByName(CF_MYCUSTOMFILELD_ENGLISH_TRANSLATION); 
log.warn("english: " + cf)

cf = customFieldManager.getCustomFieldObjectByName(CF_MYCUSTOMFILELD_FRENCH_TRANSLATION); 
log.warn("french: " + cf)

5) the output is like below

untranslated: My Custom Field
english: null
french: null

6) use script runner built-in script to flush all Jira internal caches and run the script again, the result is now as below

untranslated: null
english: My Custom Field
french: null

7) Logout and login as user admin_fr. Run the script again using this user. The result is now

untranslated: null  
english: Mon Champ Personnalisé  
french: null

8) While still logged in as user admin_fr, clear caches like above and run the script again. The result is now

untranslated: null 
english: null 
french: Mon Champ Personnalisé

9) Logout and login back with user admin_en again, and run the script again. The result is now

untranslated: null
english: null
french: My Custom Field

Expected Results

We expect consistent outputs, the cache will be rebuilt using untranslated names.

Actual Results

The cache is rebuilt according to the language of the administrator who triggered the flush.

Workaround

As a workaround, it is possible to get the full list of custom fields and check their 'untranslated name' one by one to find the correct custom field:

cf = customFieldManager.getCustomFieldObjects().find { it.getUntranslatedName() == CF_MYCUSTOMFILELD}

Please note that this may be inefficient with larger sets of custom fields.