JIRA v7.1.0#71003 (updated. see below)
When adding new gadget to system dashboard, almost all gadget titles look like following: __MSG_gadget.*.title__
See also screenshot:
2016-02-15_16-30-38.png
Additionally text for menu items and options dialog is missing when editing activity stream gadget:
2016-02-15_16-40-09.png
2016-02-15_16-41-33.png
Currently I have updated my JIRA installation to latest version:
Atlassian JIRA Project Management Software (v7.1.1#71004-sha1:d6b2c0d)
Additionally I have bought and activated the license.
Unfortunately, I still have the same problems as described above 
I believe, that the problem is caused by my configuration (local changes): I have configured secure connection over HTTPS, using the provided JIRA configuration script. This are the only changes I have done to standard installation.
By the way, I use PostgreSQL for JIRA persistence. This may also be reason if i18n problems.
UPDATE:
I have removed "Activity Stream" gadget from system dashboard and added it again. Now it has got worse. The title of gadget is now permanently "__MSG_gadget.activity.stream.title__"