PL/SQL + after login seems Not see tnsnames.ora file on my PC
What do you mean "log in by PL/SQL"? Do you mean SQL*Plus? PL/SQL is a language used to write functions and procedures - you have to be logged in already before you can call them.

The message is fairly self-explanatory: if the client and database are using different character sets, results need to be converted from one to the other before being displayed at the client, which may cause "unexpected results" (like strange characters for example).

As the database is using UTF8 it makes sense to set the client side to use the same - how to do this depends on what platform your client is on.

On Windows, you can either set an environment variable or the NLS_LANG registry setting