replaygain2: fix several issues with with call to window.set_statusbar_message

[phw]

Calls to window.set_statusbar_message had several issues:

• N_() was used, even though this neither causes translation of strings nor are translations in plugins currently supported. N_() just prepares a string for extraction. Without the use of _() somewhere this has no effect.
• Use of f-strings should be avoided here, as the function expects percent formatting. Passing in variables via f-string can lead to errors if such variables contain percentage signs (PICARD-2683)
• f-strings must also never be used in combination with translations, as this would look for a translation string matching exactly the substituted string. Instead translations must always use some kind of template string (e.g. percentage formatting or String.format())

[phw]

@complexlogic FYI

[complexlogic]

Thanks for the information, I'll keep that in mind for future plugin development