You just add it in the PL_CombatIndicator_Static.lua file after the English texts.
As for the error for Russian players, that is actually a problem for all custom localizations. There is a small bug in how the addon decides it should use English as a fallback for unsupported languages. A potential fix would look like this:
Lua Code:
local lang = GetCVar("language.2")
local str = t[lang] ~= nil and t[lang][k]
if str == nil then
It first needs to check if t[lang] even exists, otherwise it would try to access nil[k] which produces the error you see in the screenshots.
I attached the file with the necessary modifications and also added a German translation, but didn't test it. Feel free to use that.
EDIT: actually you should replace "if str == nil then" with "if not str then", since "local str = t[lang] ~= nil and t[lang][k]" will result in str being "false" if the table doesn't exist and that way you check for both "nil" and "false".