Fix English translations not used despite being preferred (flameshot-org#3822)

When specifying the preferred language on Linux, the $LANGUAGE env var
is used. https://www.gnu.org/software/gettext/manual/html_node/The-LANGUAGE-variable.html

Example:
$ LANGUAGE=en:nl flameshot

Expected:
flameshot should be in English as this is a localization that flameshot provides.
If English would not be supported, it should fall back to using Dutch (nl).

Actual:
flameshot uses Dutch translations.

Cause:
If an empty translation file for "en" is not provided, English is only
used as a fallback.
See https://bugreports.qt.io/browse/QTBUG-69196

Author: MatthiasKunnen

data/translations/Internationalization_en.ts

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!DOCTYPE TS>
+<!--
+    An empty translation file is required for the base language, or it will only be used as a
+    fallback if the user prefers other supported languages. E.g.
+    $ LANGUAGE=en:nl flameshot
+    will result in Dutch translations being used despite English being higher priority.
+    See https://bugreports.qt.io/browse/QTBUG-69196
+-->
+<TS version="2.1" language="en">
+</TS>

src/CMakeLists.txt

@@ -103,6 +103,7 @@ set(FLAMESHOT_TS_FILES
       ${CMAKE_SOURCE_DIR}/data/translations/Internationalization_cs.ts
       ${CMAKE_SOURCE_DIR}/data/translations/Internationalization_de_DE.ts
       ${CMAKE_SOURCE_DIR}/data/translations/Internationalization_el.ts
+      ${CMAKE_SOURCE_DIR}/data/translations/Internationalization_en.ts
       ${CMAKE_SOURCE_DIR}/data/translations/Internationalization_es.ts
       ${CMAKE_SOURCE_DIR}/data/translations/Internationalization_eu.ts
       ${CMAKE_SOURCE_DIR}/data/translations/Internationalization_fa.ts