I have Installation with 2 languages - English and German.
German version is "problematic"- If I define the label for Panel Get User Input, is the label for main windows still overwritten by default text (still "Benutzereingabe abrufen"):

Name:  ia_labels.png
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The change of locale-file is made in Build Phase:

custom_de before build:

# Installer.777f4947b919.installLabels.01=Test
Installer.777f4947b919.installLabels.01=Mein Test

custom_de after build:

# Installer.777f4947b919.installLabels.01=Test
Installer.777f4947b919.installLabels.01=Benutzereingabe abrufen

If is the file custom_de "read only", then is not changed
IA2010 work fine, IA2015 has the same problem as IA2013.