msgcat: add page

pages/common/msgcat.md

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+# msgcat
+
+> Concatenate and merge multiple `.po` translation files.
+> Useful in software localization pipelines to combine message catalogs with filtering options.
+> More information: <https://www.gnu.org/software/gettext/manual/html_node/msgcat-Invocation.html>.
+
+- Combine multiple `.po` files into one:
+
+`msgcat {{file1.po file2.po ...}} > {{combined.po}}`
+
+- Combine input files listed in a text file:
+
+`msgcat --files-from {{path/to/file_list.txt}} > {{combined.po}}`
+
+- Set the output encoding (e.g., UTF-8):
+
+`msgcat --to-code {{UTF-8}} {{input.po}} > {{output.po}}`
+
+- Output only unique messages (appearing in one file only):
+
+`msgcat --unique {{file1.po file2.po ...}} > {{unique.po}}`
+
+- Use the first available translation for duplicate entries:
+
+`msgcat --use-first {{file1.po file2.po ...}} > {{output.po}}`
+
+- Display help:
+
+`msgcat --help`