Language negociation issue

Problem/Motivation

opigno_group_manager.module assumes url prefixes are used for language negotiation as soon as they are defined for a language but this is not the case. It leads to a 500 error in case of prefix defined for a language, but site language being defined globally as "language selected" in negotiation.

Steps to reproduce

• Create 2 languages (EN & FR for example)
• Define EN as default language on /admin/config/regional/language
• On /admin/config/regional/language/detection use only "Selected language" and set it to FR
• This will lead to a 500 on Group manager pages

Proposed resolution

Attached patch verifies that language prefixing is actually used before assuming we should prefix path with langcode

Remaining tasks

N/A

User interface changes

N/A

API changes

N/A

Data model changes

N/A