Changed the getSerice part in all snippets and the plugin. As well as all the “instanceof \Lingua\Lingua”. Now this works - thanks a lot.
Unfortunately this brings the next issue: $allowedContexts returns empty 
And the error messages of the log are also still showing - could this be an issue of Migx 3.0.0 alpha5? MODX 3.0.3 and MIGX error
[2023-08-16 12:24:04] (ERROR @ /www/xxx/core/vendor/xpdo/xpdo/src/xPDO/xPDO.php : 667) Could not load class: Lingua from lingua
[2023-08-16 12:24:04] (ERROR @ /www/xxx/core/vendor/xpdo/xpdo/src/xPDO/xPDO.php : 1273) Problem getting service lingua, instance of class Lingua, from path