Fix source language in transifex configuration file to match the proj…

…ects language setting of the mixxx application on Transifex. This is the language selected when the project was created on Transifex, it can not be changed afterwards. The .tx/config file file is used to map files in a local repo/directory to resources in Transifex. Currently there are issues with double entries of languages in Mixxx > Preferences > Interface. This commit attempts to fix one possible cause (see https://bugs.launchpad.net/mixxx/+bug/1825644 )

.tx/config

@@ -5,7 +5,7 @@ lang_map = af_ZA: af-ZA, am_ET: am-ET, ar_AA: ar-AA, ar_AE: ar-AE, ar_BH: ar-BH,
 [mixxxdj.mixxx2-3]
 file_filter = res/translations/mixxx_<lang>.ts
 source_file = res/translations/mixxx.ts
-source_lang = en_US
+source_lang = en
 minimum_perc = 0
 type = TS