Service Desk email notification uses mixed languages

Summary

When JIRA instance uses other Default System Language, for example German (Deutsch), Service Desk notification will mix both English and German language in the email.

Environment

JIRA 6.3.15
JSD 2.3.6
JSD 2.6.0-OD-11

Steps to Reproduce

1. Install JIRA with Service Desk
2. Set Default User Preference to use Text based email notification
3. Change Indexing Language and Default Language to German (Deutsch)
4. Create a Service Desk Request on behalf of customer

Expected Results

Email notification will be fully in German language

Actual Results

There will be a mixed of English and German language in the email

Notes

1. The problem is not observed anymore in the latest JIRA and Service Desk Server version.
   JIRA 6.4.x
   JSD 2.5.0
   
2. HTML format email is also showing the translation correctly. Only the Text format is broken
3. This is still happening on Cloud instances
4. On JIRA Server the issue is happening again in JIRA 7.0.4 and JSD 3.0.4.