Fix: Error after logging out and selecting another language

[Ludy87]

Description

When logging out, the language is passed as a parameter in the URL. When changing the language, the parameter is supplemented with ?lang= and should be replaced.

after logout: http://127.0.0.1:8080/login?logout=true&lang=de_DE
after logout and language change: http://127.0.0.1:8080/login?logout=true&lang=de_DE?lang=ca_CA

Should: http://127.0.0.1:8080/login?logout=true&lang=ca_CA

Checklist:

• I have read the Contribution Guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings

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By submitting this pull request, I acknowledge and agree that my contributions will be included in Stirling-PDF and that they can be relicensed in the future under the MPL 2.0 (Mozilla Public License Version 2.0) license.

(This does not change the general open-source nature of Stirling-PDF, simply moving from one license to another license)