Loco can import Gettext PO, POT and MO files
PO files
When importing a PO file into a Loco project, the "msgid" is used as the source language and the "msgstr" as the target language translation.
This is the default, but you can override this behaviour if you need.
POT template files
Importing POT files works the same as PO files, except you should NOT specify a target language.
Specifying a target language for a POT will result in importing empty strings. That means any existing translations will be overwritten, which probably isn't what you intended.
Binary MO files
Loco is able to extract strings from compiled MO files, but note that these files won't contain metadata such as comments and references. It's always better to import a PO file if you have one.
Gettext feature support
Loco will import plural forms, message contexts, fuzzy flags, extracted comments and source code references. Some of these things have meaning within Loco, and some are simply preserved until you export them again. See more on how these features are supported.
Indexing by asset ID
The Gettext file format was designed to use a source language (usually English) to index translations in other languages. However, Loco permits you to use an alternative approach where all languages (including the source) are indexed by a generic key.
If you're following the alternative approach, you can tell Loco that your "msgid" strings are asset IDs.
To import source language translations with this method, select your source language as the "target".