Fix to ensure ttrss determines the correct language when using the API.

Example Header:
    "Accept-Language" = "de;q=1, en;q=0.9, fr;q=0.8, zh-Hans;q=0.7, zh-Hant;q=0.6, ja;q=0.5";
    Authorization = "Basic XXX";
    "Content-Type" = "application/x-www-form-urlencoded";
    "User-Agent" = "XXX";

In case the Language-Settings for the user are set to Automatic-Determine ttrss
should be using "de" but it uses "en".

It seems there is no Language-Determination for API-calls at the moment.

Adding the line in this commit fixes this issue.

To be discussed: Is this call needed for each request? Is it realistic that Acce
pt-Language changes during one session?