1. First, download the 3.8.6 PL1 patch here:
2. Delete the existing vbulletin-language.xml file from your 'install' directory. Then upload the new one to that directory. Make sure you upload this in ASCII format.
3. Next upload the two files in that patch:
4. Go into your Admin CP and run this:
Admin CP -> Languages & Phrases -> Download/Upload Languages -> Import Language XML File
Then leave the settings as they are and click on Import.
Also please note that if you have not upgraded to 3.8.6 yet, the download has already been patched.